Optical combiner designs and head mounted displays

ABSTRACT

Various embodiments involving structures and methods for illumination can be employed, for example, in projectors, head-mounted displays, helmet-mounted displays, heads-up displays, back projection TVs, flat panel displays as well as other optical systems. Certain embodiments may include prism elements for illuminating, for example, a spatial light modulator. Light may be coupled to the prism in some cases using fiber optics or lightpipes. The optical system may also include a diffuser having scatter features arranged to scatter light appropriately to produce a desired luminance profile. Ellipsoid- and parapoloid-shaped combiners may be employed in embodiments of displays such as for example head mounted displays, helmet mounted displays, and heads-up display. Other embodiments are possible as well.

PRIORITY APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. 119(e)to U.S. Provisional Patent Application No. 60/472,617, filed May 22,2003 and entitled “Combiner and Visors for Head Mounted and HelmetMounted Displays” and U.S. Provisional Patent Application No.60/472,712, filed May 22, 2003 and entitled “Optical Systems and Methodsfor Head Mounted and Helmet Mounted Displays”, which are eachincorporated herein by reference in their entirety, and to U.S.Provisional Patent Application No. 60/532,105, filed Dec. 22, 2003 andentitled “Beamsplitting Elements and Structures and Methods forIllumination”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical combiners and displays and methodsthat may be used, for example, in displays and projection systems, suchas head mounted displays, helmet mounted displays and heads-up display,etc.

2. Description of the Related Art

Optical devices for presenting information and displaying images areubiquitous. Some examples of such optical devices include computerscreens, projectors, televisions, and the like. Front projectors arecommonly used for presentations. Flat panel displays are employed forcomputers, television, and portable DVD players, and even to displayphotographs and artwork. Rear projection TVs are also increasinglypopular in the home. Cell phones, digital cameras, personal assistants,and electronic games are other examples of hand-held devices thatinclude displays. Heads-up displays where data is projected on, forexample, a windshield of an automobile or in a cockpit of an aircraft,will be increasingly more common. Helmet mounted displays are alsoemployed by the military to display critical information superimposed ona visor or other eyewear in front of the wearer's face. With thisparticular arrangement, the user has ready access to the displayedinformation without his or her attention being drawn away from thesurrounding environment, which may be a battlefield in the sky or on theground. In other applications, head mounted displays provide virtualreality by displaying graphics on a display device situated in front ofthe user's face. Such virtual reality equipment may find use inentertainment, education, and elsewhere. In addition to sophisticatedgaming, virtual reality may assist in training pilots, surgeons,athletes, teen drivers and more.

Preferably, these different display and projection devices are compact,lightweight, and reasonably priced. As many components are included inthe optical systems, the products become larger, heavier, and moreexpensive than desired for many applications. Yet such optical devicesare expected to be sufficiently bright and preferably provide highquality imaging over a wide field-of-view so as to present clear text orgraphical images to the user. In the case of the helmet or more broadlyhead mounted displays, for example, the display preferably accommodatesa variety of head positions and varying lines-of-sight. For projectionTVs, increased field-of-view is desired to enable viewers to see abright clear image from a wide range of locations with respect to thescreen. Such optical performance depends in part on the illumination andimaging optics of the display.

What is needed, therefore, are illumination and imaging optics forproducing lightweight, compact, high quality optical systems at areasonable cost.

SUMMARY OF THE INVENTION

Various embodiments are described herein. One embodiment comprises ahead mounted display for displaying images comprising: a spatial lightmodulator comprising a plurality of pixels selectively adjustable forproducing spatial patterns; a light source for illuminating said spatiallight modulator; imaging optics disposed with respect to the spatiallight modulator to receive light from said spatial light modulator forforming an image thereof; an aspheric reflective combiner substantiallyconforming to the shape of a portion of a surface selected from thegroup consisting of an ellipsoid and a paraboloid, said asphericcombiner reflecting light from said imaging optics such that said imagemay be displayed; and head gear for supporting said spatial lightmodulator, imaging optics, and combiner.

Another embodiment comprises a head mounted display for displayingimages comprising: a spatial light modulator; a light source forilluminating said spatial light modulator; first optics disposed withrespect to the spatial light modulator to receive light from saidspatial light modulator; second optics comprising an ellipsoidalcombiner having a reflective surface comprising an ellipsoidal surfacecurved in two dimensions in accordance with the curvature of anellipsoid, said ellipsoidal combiner reflecting light received from saidfirst optics; and head gear for supporting said spatial light modulatorand first and second optics.

Another embodiment comprises a method of forming an image in the eye ofa wearer of a head mounted display, said method comprising: illuminatinga spatial light modulator in said head mounted display; collecting lightfrom said spatial light modulator with projection optics having an exitpupil; and reflecting said collected light from an ellipsoidal reflectorinto the eye of the wearer to form an image at the eye, said ellipsoidalreflector having a shape substantially conforming to a portion of anellipsoid.

Another embodiment comprises a heads-up display for displaying imagescomprising: a spatial light modulator for producing spatial patterns; alight source for illuminating said spatial light modulator; imagingoptics disposed with respect to the spatial light modulator to receivelight from said spatial light modulator for forming an image thereof,and an optical combiner substantially conforming to the shape of aportion of a conic surface having a conic constant, k, where −1≦k≦0,said optical combiner reflecting light from said imaging optics suchthat said image may be displayed.

Another embodiment comprises a method of forming an image in a person'seye, said method comprising: illuminating a spatial light modulator;collecting light from said spatial light modulator with projectionoptics having an exit pupil; and reflecting said collected light from anreflector into the eye to form an image at the eye, said reflectorhaving a shape substantially conforming to a portion of an paraboloid.

Another embodiment comprises a display for displaying images comprising:a spatial light modulator comprising a plurality of pixels selectivelyadjustable for producing spatial patterns; a light source forilluminating said spatial light modulator; imaging optics disposed withrespect to the spatial light modulator to receive light from saidspatial light modulator for forming an image thereof; a combinerreflecting light from said imaging optics such that said image may bedisplayed; wherein said imaging optics comprise first and second groupsof optical elements having respective first and second optical differentoptical axes, said first group comprising a plurality of opticalelements and said second group comprising no more than one opticalelement. In certain embodiments, this one lens in the second groupcomprises a meniscus lens. In various embodiments, the lenses in thefirst and second groups are plastic lenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a display apparatus comprising abeamsplitter disposed in front of a spatial light modulator that directsa beam of light to the spatial light modulator to provide illuminationthereof;

FIG. 2 is a perspective view of a projection system comprising anoptical apparatus similar to that depicted schematically in FIG. 1;

FIG. 3 is a schematic representation of a preferred display apparatuscomprising a “V” prism for illuminating a spatial light modulator;

FIG. 4 is a perspective view of an optical system for a rear projectionTV comprising a “V” prism such as shown in FIG. 3 disposed between apair of light sources for illuminating a spatial light modulator;

FIG. 5 is a perspective view of a prism device having a pair ofreflective surfaces for providing illumination of a display, whereinlight is coupled into the prism via light propagating conveyances;

FIG. 6 is a perspective view of a prism element having four input portsfor receiving light from four integrating rods and four reflectivesurfaces for reflecting the light input through the four input ports;

FIG. 7 is a perspective view of another prism structure having fourinput ports for receiving light and four reflecting faces comprisingwire grid polarizers for reflecting polarized light input into the inputports;

FIG. 8A is a cross-sectional view of the prism structure shown in FIG. 7along the line 8A-8A;

FIG. 8B is a top view of the prism structure depicted in FIGS. 7 and 8Ashowing the four triangular faces and wire grid polarizers forreflecting polarized light input into the four ports of the prismstructure;

FIGS. 9A and 9C are perspective views of other prism structures havingmultiple input ports for receiving light and a reflecting surface forreflecting polarized light input into the input ports;

FIGS. 9B and 9D are a cross-sectional views of the prism structuresshown in FIGS. 9A and 9D along the lines 9B—9B, and 9D—9D respectively;

FIG. 10 is a schematic representation of an illuminating systemcomprising a “V” prism further comprising a plurality of light sources,as well as beamshaping optics and a diffuser for each of two inputports;

FIG. 11 is a schematic representation of an optical fiber bundle splitto provide light to a pair of input ports of an illumination device suchas the prism shown in FIG. 10;

FIGS. 12 and 13 are schematic representations of the illuminanceincident on two respective portions the spatial light modulator whereinthe illuminance has a Gaussian distribution;

FIG. 14 is plot on axes of position (Y) and illuminance depicting aGaussian distribution;

FIG. 15 is a schematic representation of the illuminance distributionacross the two portions of the spatial light modulator which isilluminated by light reflected from the respective reflecting surfacesof the “V” prism;

FIGS. 16 and 17 are schematic representations of the illuminanceincident on the two portions of the spatial light modulator of the “V”prism wherein the central peak is shifted with respect to the respectiveportions of the spatial light modulator;

FIG. 18 is a schematic representation of the illuminance having a “flattop” distribution incident on one portion of the spatial lightmodulator;

FIG. 19 is a plot on axes of position (Y) and illuminance depicting a“top hat” distribution;

FIG. 20 is a cross-sectional schematic representation of a diffuserscattering light into a cone of angles;

FIG. 21 is a plot on axes of angle, θ, and intensity illustratingdifferent angular intensity distributions that may be provided bydifferent types of diffusers;

FIG. 22 is a schematic illustration of a field-of-view for a displayshowing a non-uniformity in the form of a stripe at the center of thefield caused by the V-prism;

FIG. 23 is a cross-sectional view of the V-prism schematicallyillustrating the finite thickness of the reflective surfaces of the Vprism that produce the striped field non-uniformity depicted in FIG. 22;

FIG. 24 is a cross-sectional view of a wire grid polarizer comprising aplurality of strips spaced apart by air gaps;

FIG. 25 is a cross-sectional view of a wire grid polarizer comprising aplurality of strips with glue filled between the strips;

FIG. 26 is a cross-sectional view of a wire grid polarizer comprising aplurality of strips and a MgF overcoat formed thereon;

FIGS. 27A-27G are cross-sectional views schematically illustrating oneembodiment of a process for forming a V-prism comprising a pair of wiregrid polarization beamsplitting surfaces;

FIG. 28 is a cross-sectional view of a wedge shaped optical element forproviding correction of astigmatism and coma that is disposed betweenthe “V” prism and the spatial light modulator;

FIG. 29 is a cross-sectional view of a “V” prism having a wedge shapethat includes correction of astigmatism and coma;

FIG. 30 is a plot on axes of position (Y) versus illuminance on thespatial light modulator for a wedge-shaped prism used in combinationwith different type diffusers;

FIG. 31 is a plot of the illuminance distribution across the spatiallight modulator provided by a wedge-shaped “V” prism;

FIG. 32 is a histogram of luminous flux per area (in lux) thatillustrates that the luminous flux per area received over the spatiallight modulator is within a narrow range of values;

FIG. 33 is a plot of the illuminance distribution across the spatiallight modulator provided by a “V” prism in combination with a wedgeseparated from the “V” prism by an air gap such as shown in FIG. 28;

FIG. 34 is a histogram of luminous flux per area (in lux) thatillustrates that the luminous flux per area received over the spatiallight modulator is within a narrow range of values;

FIG. 35 is a schematic representation of a V-prism together with anX-cube;

FIG. 36 is a schematic representation of a V-prism together with aPhilips prism;

FIG. 37 is a schematic representation of a configuration having reduceddimensions that facilitates compact packaging;

FIG. 38 is a schematic representation of a configuration for providingnon-constant illuminance at the spatial light modulator;

FIG. 39 shows graded illuminance across the spatial light modulator;

FIG. 40 is a plot on axis of illuminance versus position (Y) showingthat the illuminance across the spatial light modulator increases fromone side to another;

FIG. 41 is a cross-sectional view of a diffuser that scatters lightdifferent amounts at different locations on the diffuser;

FIG. 42 shows three locations on a diffuser that receive differentlevels of luminous flux corresponding to different illuminance values(I₁, I₂, and I₃) and that scatter light into different size cone angles(Ω₁, Ω₂, and Ω₃) such that the luminance at the three locations (L₁, L₂,and L₃) is substantially constant;

FIG. 43 is a plot of luminance across the spatial light modulator, whichis substantially constant from one side to another;

FIG. 44 is a histogram of luminous flux per area per solid angle (inNits) that illustrates that the luminous flux per area per solid anglevalues received over the spatial light modulator are largely similar;

FIG. 45 is a cross-sectional view schematically showing a light box anda plurality of compound parabolic collectors optically connected theretoto couple light out from the light box; and

FIGS. 46-56 are schematic representations of displays such as headmounted displays.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To present graphics or other visual information to a viewer, imagesand/or symbols, e.g., text or numbers, can be projected onto a screen ordirected into the viewer's eye. FIG. 1 schematically illustrates adisplay 10 disposed in front of a viewer 12 (represented by an eye). Ina preferred embodiment, this display 10 includes a spatial lightmodulator 14 that is illuminated with light 16 and imaged with imagingor projection optics 18. The spatial light modulator 14 may comprise,for example, a reflective polarization modulator such as a reflectiveliquid crystal display. This liquid crystal spatial light modulatorpreferably comprises an array of liquid crystal cells each which can beindividually activated by signals, e.g., analog or digital, to produce ahigh resolution pattern including characters and/or images. Moregenerally, the spatial light modulator may comprise an array ofmodulators or pixels that can be selectively adjusted to modulate light.The projection optics 18 may, for example, project the image to infinity(or a relatively large distance) or may form a virtual image that may beimaged onto the retina by the eye. Such a display may be employed, forexample, in a television or head mounted display.

To illuminate the LCD spatial light modulator 14, a beamsplitter 20 isdisposed in front of the LCD. The beamsplitter 20 has a reflectivesurface 22 that reflects the beam of light 16 introduced through a side24 of the beamsplitter toward the LCD 14. Reflections from the LCD 14pass through the reflective surface 22 on another pass and exit a frontface 26 of the beamsplitter 20. The imaging optics 18 receives the lightfrom the beamsplitter 20 and preferably images the pattern produced bythe LCD display 14 onto the retina of the viewer's eye 12.

Preferably, the light entering the side 24 of the beamsplitter 20 ispolarized light and the beamsplitter comprises a polarizationbeamsplitter. In such a case, the reflective surface 22 may preferablycomprise a polarization dependent reflective surface that reflects lighthaving one polarization and transmits light having another polarizationstate. The cells within the LCD spatial light modulator 14 also may forexample selectively rotate the polarization of light incident on thecell. Thus, the state of the LCD cell can determine whether the lightincident on that cell is transmitted through the reflective surface 22on the second pass through the beamsplitter 20 based on whether thepolarization is rotated by the cell. Other types of liquid crystalspatial light modulators may also be used as well.

A perspective view of similar type of optical apparatus 30 is shown inFIG. 2. This device 30 may also include a projection lens 18 and may beemployed as a projector to project a real image of the spatial lightmodulator 14 onto a screen 31. The beamsplitter 20 may comprise a prismsuch as a polarization beamsplitting prism, and in certain preferredembodiments, the beamsplitter may comprise a multi-layer coatedbeamsplitting prism comprising a stack of coating layers that providepolarization discrimination as is well known in the art. MacNeille-typepolarizing cubes comprising a cube such as shown in FIG. 1 with amultilayer coating on a surface tilted at an angle of about 45° may beused; however, the field-of-view may be limited by dependence of theefficiency of the multilayer on the angle of incidence. If instead ofthe conventional multilayer coating employed on MacNeille beamsplittingcubes, the coating layers comprise birefringent layers that separatepolarization based on the material axis rather than the angle ofincidence, effective performance for beams faster than f/1 can beobtained. Such birefringent multilayers may be available from 3M, St.Paul, Minn.

Alternative beamsplitters 20 may be employed as well. Examples of somealternative polarization beamsplitters that separate light into twopolarization states include crystal polarizers and plate polarizers.Advantageously, crystal polarizers have a relatively high extinctionratio, however, crystal polarizers tend to be heavy, relativelyexpensive, and work substantially better for relatively slow beams withlarger f-numbers (f/#). Image quality is predominantly better for onepolarization compared to another. Plate polarizers can comprisemulti-layer coatings that are applied on only one side of a plateinstead of in a cube. Plate polarizers are light and relativelyinexpensive. However, image quality is also primarily higher for onepolarization state, and with plate polarizers, the image quality isdegraded substantially for speeds approaching f/1. Other types ofpolarizers such as photonic crystal polarizers, and wire-grid polarizersmay be employed as well. Photonic crystal polarizers comprise a stack oflayers that forms a photonic crystal that can be used to discriminatepolarizations. Photonic crystal polarizers are available from PhotonicLattice Inc., Japan. Photonic crystal polarizers have theoreticallyexcellent fields-of-view and wavelength acceptance; however, photoniccrystal polarizers are fabricated using expensive lithographicprocesses. Wire grid polarizers comprise a plurality of wires alignedsubstantially parallel across a planar surface. These wire grids mayalso discriminate polarization. Wire grid polarizers may be available,e.g., from NanoOpto Corporation, Summerset, N.J., as well as Moxtek,Inc., Orem, Utah. Wire grid polarizers have good extinction intransmission; however, these polarizers are somewhat leaky inreflection. Aluminum used to form the wire grid also tends to havehigher absorption than dielectric materials. Nevertheless, wire gridpolarizers are preferred for various embodiments of the invention.

As discussed above, multi-layer coatings comprising a plurality ofbirefringent layers in cube polarizers work well for beams faster thanf/1 and provide high image quality for both polarizations. Wire gridpolarizers and photonic crystal polarizers, may replace the birefringentmultilayers in the beamsplitter cube in preferred embodiments. The cubeconfiguration, however, depending on the size, can be heavy. Thebeamsplitter 20 shown in FIGS. 1 and 2 comprises a beamsplitter cubehaving sides of approximately equal length. Similarly, the beamsplitter20 has a size (e.g., thickness, t) greater than the width, w, of thespatial light modulator 14.

As shown in FIG. 2, a light source 32 is disposed with respect to thepolarization beamsplitter 20 to introduce light into the beamsplitter toilluminate the spatial light modulator 14. The beamsplitter 20 includesone port for receiving light. The light is introduced into thebeamsplitter 20 through the side 24. The reflective surface 22 is slopedto face both this side 24 and the LCD display 14 such that light inputthrough the side 24 of the beamsplitter is reflected toward the LCDdisplay. This reflective surface 22 may comprise a planar surface tiltedat an angle of between about 40 and 50 degrees with respect to the side24 of the beamsplitter but may be inclined at other angles outside thisrange as well. The illuminated LCD display can be imaged with theimaging optics 18. The imaging optics 18 may comprise a projection lensthat is relatively large and heavy to accommodate a sufficiently largeback focal distance and a sufficiently large aperture through thebeamsplitter cube 20 to the spatial light modulator 14.

Beamsplitters with other dimensions or having other geometries andconfigurations may also be employed as well. A variety of novelbeamsplitters and optical systems using beamsplitters are describedherein. In one exemplary embodiment of the invention, for example, byincluding two or more ports, the thickness of the beamsplitter may bereduced. Such a design is illustrated in FIG. 3. FIG. 3 shows a display40 comprising a beamsplitter device 42 having two ports 44, 46 forreceiving two beams of light 48, 50. The display 40 further comprises aspatial light modulator 52 and imaging optics 54 for imaging the spatiallight modulator. As discussed above, the spatial light modulator 52 maycomprise a liquid crystal spatial light modulator comprising an array ofliquid crystal cells. These liquid crystal cells may be selectivelycontrolled accordingly to data or video signals received by the spatiallight modulator.

The beamsplitter device 42 may comprise a prism element comprising glassor plastic or other materials substantially transparent to the incidentlight 48, 50. The prism element 42 shown has two input faces 56, 58 forreceiving the two beams of light 48, 50, respectively. In the embodimentillustrated in FIG. 3, these two input surfaces 56, 58, are parallel andcounter-opposing, disposed on opposite sides of the prism. Similarly,the two input ports 44, 46 are oppositely directed, the optical path ofthe corresponding light beam being directed along substantially oppositedirections. Although the input ports 44, 46 are oriented 180° withrespect to each other, other configurations where, for example, theports are directed at different angles such as 30°, 40°, 60°, 72°, 120°etc., and angles between and outside these ranges are possible. The twoinput faces 56, 58 are preferably substantially optically transmissiveto the light 48, 50 such that the light can be propagated through theprism 42.

The prism element 42 also has two reflecting surfaces 60, 62 thatreflect light received by the two ports 44, 46 toward a first(intermediate) output face 64 and onto the spatial light modulator 52.The two reflecting surfaces 60, 62 are sloped with respect to the inputand output faces 56, 58, 64 such that light input through the inputfaces is reflected to the output face. In one preferred example, thereflecting surfaces 60, 62 are inclined at an angle of between about 40to 50 degrees with respect to the input faces 56, 58 and at an angle ofbetween about 40 to 50 degrees with respect to the first output face 64.The angle of inclination or declination, however, should not be limitedto these angles.

The two reflective surfaces 60, 62 are also oppositely inclined. In theexample shown in FIG. 3, the reflective surfaces 60, 62 slope from acentral region of the output face 64 to the respective opposite inputfaces 56, 58. The reflecting surfaces 60, 62 meet along a line or edge66 in the central region of the output face 64, and may be coincidentwith the output face 64. This configuration, however, should not beconstrued as limiting as other designs are possible. The prism 42 may bereferred to herein as a “V” prism in reference to the “V” shape formedby the reflective surfaces 60, 62 that are oppositely inclined orsloping and that preferably converge toward the vertex (or apex) 66located in the central region of the output face 64.

Preferably, each of the reflective surfaces 60, 62 comprises apolarization —dependent reflective surface that reflects light havingone polarization and transmits light having another polarization state.For example, the reflective surfaces 60, 62 may each reflect thes-polarization state and transmit the p-polarization state or viceversa. Alternative configurations are possible and the reflectivesurfaces 60, 62 may be designed to reflect and transmit other states aswell. In various preferred embodiments, the reflective surfaces 60, 62are formed using multi-layered birefringent coatings or wire grids asdescribed above.

The “V” prism 42 can therefore be said to be a polarizationbeamsplitter, as this prism device splits beams having differentpolarizations. Preferably, however, light entering the sides of thebeamsplitter 42 is polarized light. In such a case, the reflectivesurfaces 60, 62 are preferably selected to reflect the light beams 48,50 introduced through the respective sides 56, 58 of the beamsplitter42. The input beams 48, 50 propagating along paths oppositely directedand parallel to the Y-axis (as shown in FIG. 3) are redirected alongsimilarly directed optical paths parallel to the Z-axis towards the LCD52. The spatial light modulator 52 is also preferably a reflectivedevice. Accordingly, light from both input beams 48, 50 traveling towardthe liquid crystal array 52 is preferably reflected in an oppositedirection along a path parallel to the Z-axis back to the reflectivesurfaces 60, 62.

The cells within the LCD spatial light modulator 52 also preferablyselectively rotate the polarization of light incident on the cell. Thus,reflections from the LCD 52 will pass through the reflective surfaces60, 62 on another pass and exit a front face 68 of the beamsplitter 42.In this manner, the state of the LCD cell can determine whether thelight incident on that cell is transmitted through the reflectivesurface 60, 62 on the second pass through the beamsplitter 42 based onwhether the polarization is rotated by the respective cell. Highresolution patterns such as text or images can thereby be produced byindividually activating the liquid crystal cells using, for example,electrical signals. Other types of spatial light modulators may be used.These spatial light modulators may be controlled by other types ofsignals. These spatial light modulators may or may not comprise liquidcrystal, may or may not be polarization dependent, and may or may not bereflective. For example, transmissive spatial light modulators may beemployed in alternative embodiments. The type of spatial lightmodulator, however, should not be restricted to those recited herein.

The imaging optics 54 images the spatial light modulator 52. The imagingoptics 54 enables patterns created by the modulated liquid crystal array52 to be formed on the retina of the viewer or in other embodiments, forexample, on a screen or elsewhere.

The addition of an input port 46 and a corresponding reflective surface62 permits the beamsplitting element 42 to have a smaller thickness, t.As shown in FIGS. 1 and 3, the respective prism elements 20, 42 havewidths, w. The ratio of the thickness to the width (t/w) is less for the“V” prism 42 as a smaller thickness is required to accommodate a givenprism width, w. Similarly, a smaller thickness, t, is needed toilluminate a spatial light modulator 14, 52 having a given width, w.

The width of the spatial light modulator 14, 52 may be, for example, ½to 1 inch (13 to 25 millimeters) on a diagonal. The thickness of theprism 42 may be between about ¼ to ½ inch (6 to 14 millimeters).Accordingly, the input faces 56, 58 and reflective surfaces 60, 62, maybe between about ⅓×½ inch (9×12 millimeters) to about ⅔×1 inch (18×24millimeters), respectively. A beam 1 inch (25 millimeters) diagonal maybe used to illuminate the spatial light modulator 14, 52. Otherdimensions outside these ranges may be used and should not be limited tothose specifically recited herein. Also, although the shape of thespatial light modulator 42 as well as the shape of the input ports 56,58 and the reflective surfaces 60, 62 may be square or rectangular inmany embodiments, other shapes are possible.

As discussed above, adding additional ports 46 such as provided by the“V” prism 42 may advantageously yield a smaller, lighter, more compactillumination system. For example, the thickness and mass of the “V”prism polarization beamsplitting element 42 may be about ½ that of apolarization beamsplitting cube 20 for illuminating a same size area ofthe spatial light modulator 14, 52 specified by the width, w. Similarly,the back focal distance of the projection lens or imaging optics 54 maybe shortened. As a result, the imaging optics 54 used in combinationwith the “V”-prism can be reduced in size (e.g., in diameter) incomparison with the imaging optics 18 used in combination with a prismcube 20 in a display having a similar f-number or numerical aperture.Reduced size, lower cost, and possibly improved performance of theimaging optics 54 may thus be achieved.

In one preferred embodiment, the “V”-prism 42 comprises a square prismelement comprising three smaller triangular prisms having a triangularshape when viewed from the side as shown in FIG. 3. A method offabricating such a “V” prism 42 is discussed below with reference toFIGS. 27A-27G. The prisms 42 may have polarization beamsplittingcoatings such as multiple birefringent layers to create selectivelyreflective surfaces 60, 62 that separate polarization states. Such a“V”-prism 42 preferably performs well for f-numbers down to about f/1and lower. In other embodiments, the polarization beamsplitting surfaces60, 62 may comprise wire grid polarizers or photonic crystalpolarization layers, for example.

FIG. 4, shows an illumination engine 53 for a rear projection television(which may be, e.g., an HDTV) comprising a “V” prism 42. The “V” prism42 comprises a pair of polarization beamsplitting cubes 70, 72 arrangedsuch that the reflective surfaces 60, 62 are oppositely inclined andthus face different directions. Accordingly, as described above, lightinput from the two oppositely directed input ports 44, 46 can bereflected through the output face 64 on each of the beamsplitters 70, 72for example, to a liquid crystal spatial light modulator 52. FIG. 4depicts two sources of illumination 74, 76 coupling light in the twooppositely facing ports 44, 46 located on opposite sides of the prismelement 42. This light 48, 50 following oppositely directed pathsparallel to the Y-axis, is reflected from the sloping reflectivesurfaces 60, 62 along a path parallel to the Z-axis toward the spatiallight modulator 52. The two polarization beamsplitters 70, 72 in thedevice 42 may be secured in place using optical contact, cement,adhesive, clamps, fasteners or by employing other methods to positionthe two cubes appropriately. Preferably, these two polarization cubes70, 72 are adjoining such that the reflective surfaces 60, 62 are insufficiently close proximity to illuminate the spatial light modulator52 without creating a dark region between the two polarization cubes.The “V”-prism 42 may be formed in other ways as well.

The illumination engine 53 shown in FIG. 4 further includes a supportassembly 55 for supporting the “V” prism and the sources of illumination74, 76. Although this support assembly 55 is shown as substantiallyplanar, the support assembly need not comprise a board or planarsubstrate. Other approaches for supporting the various components may beused and the specific components that are affixed or mounted to thesupport structure may vary. The support structure 55 may for examplecomprise a frame for holding and aligning the optics. Walls or a base ofthe rear projection TV may be employed as the support structure 55. Theexamples describe herein, however, should not be construed as limitingthe type of support used to support the respective system.

Each of these illumination sources 74, 76 comprise an LED array 57 andfirst and second fly's eye lenses 59, 61 mounted on the support assembly55. The fly's eye lenses 59, 61 each comprise a plurality of lenslets.In various preferred embodiments, the first and second fly's eye lenses59, 61 are disposed along an optical axis from the LED array 57 to thespatial light modulator 52 through the reflective surfaces 60, 62 withsuitable longitudinal separation. For example, the LED array 57 isimaged by the first fly's eye lens 59 onto the second fly's eye lens 61,and the first fly's eye lens is imaged by the second fly's eye lens ontothe spatial light modulator 52. In such embodiments, the first fly's eyelens 59 may form an image of the LED array 57 in each of the lenslets ofthe second fly's eye lens 61. The second fly's eye lens 61 formsoverlapping images of the lenslets in the first fly's eye lens 59 ontothe spatial light modulator 52. In various preferred embodiments, thefirst fly's eye 59 comprises a plurality of elongated or rectangularlenselets that are matched to the portion of the spatial light modulator52 to be illuminated by the LED array 57.

The illumination engine 53 further comprises imaging or projectionoptics 54 for example for projecting an image of the LCD 52 onto ascreen or display or directly into an eye. The illumination engine 53depicted in FIG. 4 is shown as part of a rear projection TV having aflat projection screen 63 and a tilted reflector 65 for forming theimage on the screen for the viewer to see. One or more additionalreflectors may be employed to reorient the image or to accommodateillumination engines 53 having output in different directions. As the“V” prism 42 may have reduced thickness in comparison to a polarizationcube for illuminating a similarly dimensioned region of the spatiallight modulator 52, the imaging or projection optics 54 in theillumination engine 53 may be scaled down in size in comparison with asystem having an identical f-number or numerical aperture.

Other configurations and designs for providing illumination arepossible. FIG. 5, for example, depicts a prism device 80 comprising apair of reflective surfaces 82, 84 oriented differently than thereflective surfaces in the “V” prism 42. The prism element 80 showncomprises a pair of polarization beamsplitting cubes 86, 88 with thereflective surfaces 82, 84 formed using wire grid polarizers, althoughMacNeille-type prisms could be employed in other embodiments. The wiregrid polarizers comprise an array of elongated strips or wires arrangedsubstantially parallel. In various preferred embodiments, theseelongated strips comprise metal such as aluminum. The wire gridpolarizers reflect one linear polarization and transmit anotherorthogonal linear polarization. Alternative embodiments may employ othertypes of polarizers such as polarizers formed from multiple birefringentlayer coating as well as photonic crystal polarizers.

The prism element 80 has two ports 90, 92 on different sides of theprism element. Light piping 95 is shown in phantom in FIG. 5 asdirecting illumination from a light source (not shown) through tworespective input faces 98, 100, one on each of the polarizationbeamsplitting cubes 86, 88. The light piping 95 may comprise sidewalls97 that form conduits or conveyances with hollow channels 99 thereinthrough which light propagates. Preferably, the inner portions 101 ofthe conduits are reflecting, and may be diffusely reflecting in certainpreferred embodiments, such that light propagates through the innerchannel of the light piping 95 from the light source to the input faces98, 100 of the prism element 80. The light piping 95 shown in FIG. 5branches into two arms 103 a, 103 b that continue toward the two inputfaces 98, 100. Preferably, the two arms 103 a, 103 b have suitabledimensions and reflectivity of the respective sidewalls 97 to providesubstantially equal illumination at the two input faces 98, 100. Invarious preferred embodiments, the light piping 95 may be shaped (e.g.,molded) to accommodate or conform to the other components or to fit intoa particular space in a device, such as a helmet-mounted display or,more broadly, a head-mounted display. (As used herein helmet-mounteddisplays, which accompany a helmet, are one type of head-mounteddisplay, which may or may not be mounted on a helmet.)

Each of the reflective surfaces 82, 84 in the prism device 80 isoriented at an angle with respect to the input faces 98, 100 and anoutput face 102. The angle with respect to the output face 102 may be,for example, between about 40 to 50 degrees or outside these ranges. Thereflective surfaces 82, 84 in this prism element 80, however, facedifferent directions on different sides of the prism element than thereflective surfaces 60, 62 in the “V” prisms 42. For example, one of thereflective surfaces 84 is oriented to receive light propagating along anoptical path parallel to the X-axis and to reflect the light along anoptical path parallel to the Z-axis. The other reflective surface 82 isoriented to receive light propagating along an optical path parallel tothe Y-axis and to reflect the light along an optical path parallel tothe Z-axis. Accordingly, the two reflective surfaces 82, 84 facedifferent directions, here 90° apart. Ports directed along otherdirections also may be employed.

A range of other configurations are possible wherein a pair ofreflective surfaces are provided. Preferably, these reflective surfacesare inclined to reflect light input into the prism element 80 from oneof the side surfaces along a common direction. Different input sides canbe used as the input surfaces in different embodiments. For example, theside surfaces can be oppositely facing or can be oriented 90 degreeswith respect to each other or at different angles with respect to eachother. The reflective surfaces can be planar and square or rectangularas shown in FIG. 5 or may have different shapes. The reflective surfacescan be tilted substantially the same amount or can be inclined ordeclined or be angled different amounts. The reflective surfaces canalso be inclined in different directions. Still other configurations areconsidered possible and should not be limited to those specificallydescribed herein as variations can be suitably employed consistent withthe teaching disclosed herein.

FIG. 6 shows a square prism element 110 with four input ports 112 onfour separate sides of the square prism. Four light sources 114 coupledto rectangular integrating rods 116 are also depicted. The rectangularintegrating rods 116 may comprise hollow conduits with inner sidewallsthat are reflecting, possibly diffusely reflecting. In alternativeembodiments, the rectangular integrating rods 116 are not hollow butinstead comprise material such as glass, crystal, polymer, that issubstantially optically transmissive and that is shaped to providereflecting sidewalls. Light propagates through this material or throughthe hollow conduit reflecting multiple times from the sidewalls of theintegrating rod 116. The multiple reflections preferably provide mixingthat homogenizes the output, preferably removing bright spots or othernon-uniformities. In some embodiments, the integration rods 116 have asquare or rectangular cross-section orthogonal to respective opticalaxes extending lengthwise therethrough. Such cross-sections aredesirable for illuminating a square or rectangular region on the spatiallight modulator. Other shapes are also possible. In various preferredembodiments, the cross-section is elongated in one direction, as is arectangle. Also, although rectilinear shaped integrating rods 116 areshown, curvilinear structures may be employed as well. Lightpipes thatfollow a curve path including, for example, fiber bundles, large corefibers, and other substantially flexible lines that may be bent may beemployed. Alternatively, rigid but curved lightpipes may be employed aswell in alternative embodiments.

The four input ports 112 include input surfaces 118 each forming anoptical path to one of four respective reflecting surfaces 120. The fourports 112 and input surfaces 118 face four different directions outwardfrom the four sides of the square prism 110. The reflective surfaces 120also face four different directions. These reflective surfaces 120 aretilted toward an output face 124, which is depicted in FIG. 6 as underor behind the prism element 110. Accordingly, light received by the fourinput surfaces 118 is deflected downward in FIG. 6 toward the outputface 124 where a reflective LCD module (not shown) may be located.Preferably, these reflective surfaces 120 are polarization splittingsurfaces, and the light input is polarized such that the light reflectstoward the output face 124. The prism element 110 may be formed fromfour adjoining beamsplitting cubes appropriately oriented.

Four polarizers may be inserted between the light sources 114 or theintegrating rods 116, and the input faces 118. These polarizers may bereferred to herein as pre-polarizers. The polarizers preferably ensurethat substantially all the light reaching the input faces 118 hassuitable polarization such that this light is reflected by thepolarization splitting reflective surfaces 120.

Another embodiment of a square prism element 150 having four input ports152 is illustrated in FIG. 7. This prism element 150 includes four faces160 where light can be input and four triangular reflective surfaces 170that are similarly inclined toward an apex region 175 such that lightinput through the input face 160 is reflected upward and out an outputsurface 178 as shown in FIGS. 7, 8A, and 8B. A spatial light modulator(not shown) such as a reflective liquid crystal array device or otherreflective modulator assembly may be located adjacent the output surface178 to reflect light back into the prism 150 via the output face 178. Aside sectional view as well as a top view are depicted in FIGS. 8A and8B. The adjacent triangular reflective surfaces 170 are preferablyadjoined to each other along edges 180 that are inclined toward the apexregion 175. In the orientation shown in FIGS. 7, 8A, and 8B, the fourreflective surfaces 170 appear to form a pyramid-shaped surface. Thefour input ports 152 face four different directions outward from thesquare prism 150. The four triangular reflective surfaces 170 also facefour different directions. Preferably, the four reflective surfaces 170comprise polarization splitting surfaces that reflect one polarizationstate and transmit another polarization state. These four surfaces mayreflect similar or different polarizations. Preferably, polarized lightis coupled into the input ports 152 such that the light is reflectedfrom the polarization splitting reflective surfaces 170. Thesepolarization splitting interfaces 170 may be formed using multilayeredcoatings, grid polarizers, and photonic crystals, as described above aswell as other types of polarizers both known and yet to be devised. Gridpolarizers 190 comprising arrays of parallel metal strips are shown inFIGS. 7, 8A, and 8B. The size of these grid polarizers 190 and the metalstrips forming the polarizers are exaggerated in the schematic drawingspresented.

Another embodiment of a prism element 150 having multiple input ports152 is illustrated in FIG. 9A. This prism element 150 comprises acircularly symmetric prism. The prism element 150 includes input faces160 where light can be input and reflective surfaces 170 that aresimilarly inclined toward an apex region 175 such that light inputthrough the input faces 160 is reflected upward and out an outputsurface 178 as shown in FIGS. 9A and 9B. A spatial light modulator (notshown) such as a reflective liquid crystal modulator assembly may belocated adjacent the output surface 178 to reflect light back into theprism 150 via the output face 178. A side sectional view is depicted inFIG. 9B. The reflective surfaces 170 are preferably inclined toward theapex region 175. In the orientation shown in FIGS. 9A-9B, the reflectivesurfaces 170 appear to form a conical-shaped surface. The surface 170 iscircularly symmetric about an axis 179 through the apex 175. The inputports 152 face different directions outward from the circular prism 150.The reflective surfaces 170 also face different directions. As shown inthe cross-section in FIG. 9B, the surface is curved along a directionparallel to the axis 179. The curvature, slope, concavity may vary.Other variations in the curvature may be included. Other types ofsurfaces of revolution providing inclined reflective surfaces may alsobe employed. FIGS. 9C and 9D depict a prism 150 having a reflectivesurface 170 shaped like a cone. Instead of having a curvature thatvaries along the axis of rotation, the slope is substantially constant.The linear incline of this reflective surface 170 is depicted in thecross-section shown in FIG. 9D. The surfaces shown in FIGS. 9A and 9Chave shapes conforming to the shape of surfaces of revolution about theaxis 179. Polarization beamsplitting surfaces having shapes that conformto portions of such surfaces of revolution are also possible. Also, thecurve that is rotated to form the surface of revolution for thecorresponding shape may be irregular, yielding differently shapedsurfaces. Other shapes are possible for the reflective surfaces 170.

Preferably, the reflective surfaces 170 comprise polarization splittingsurfaces that reflect one polarization state and transmit anotherpolarization state. Preferably, polarized light is coupled into theinput ports 152 such that the light is reflected from the polarizationsplitting reflective surfaces 170. These polarization splittinginterfaces 170 may be formed using multilayered coatings, gridpolarizers, and photonic crystals, as described above as well as othertypes of polarizers both known and yet to be devised.

The prism elements preferably comprise glass or other materialsubstantially transmissive to the light input into the input ports.Examples of optically transmissive materials that may be employedinclude BK7 and SFL57 glass. Other materials may be employed as well andthe prism should not be limited to those transmissive materialsspecifically recited herein. These prism elements need not be limited tosquare configurations. Other shapes and sizes such as for examplerectangular, hexagonal, etc. can be employed. Other techniques forreflecting one polarization state and transmitting another polarizationstate can be used as well. These reflective surfaces, for example, maycomprise polarization plates in various embodiments.

As discussed above, the resultant illumination device is thinner andthus provides for lighter, more compact designs. Lower cost and higherperformance may also be achieved. Smaller projection optics with shorterback focal length may also be employed.

An optical apparatus 200 is depicted in FIG. 10 comprising a “V” prism202 that is optically coupled to an array of light emitting diodes(LEDs) 204 via optical fiber lines 206 to first and second input ports208, 210. Such an optical apparatus 200 may be included in ahead-mounted display such as a helmet-mounted display and may beenclosed in a housing and supported on a support structure (both notshown). The fiber lines 206 are considered to be a particular type oflight pipe which include incoherent fiber bundles, coherent fiberbundles, large core optical fibers, hollow conduits, or other types oflight pipes. Optical fiber lines 206 advantageously offer flexibility,for example, for small compact devices and designs where packagingrequirements restrict size and placement of components. The LED array204 comprises three LEDs 212, which may comprise for example red, green,and blue LEDs. The three LEDs 212 are depicted coupled to the threeoptical fiber lines 206. Each of the three optical fiber lines 206 issplit into a pair of separate first and second fiber lines 206 a, 206 b.The first fiber line 206 a associated with each of the three LEDs isoptically coupled to the first input port 208 of the “V” prism 202. Thesecond fiber line 206 b associated with each of the three LED 206 isoptically coupled to the second input port 210 of the “V” prism 202.Light from each of the LEDs 212 can therefore be distributed to bothports 208, 210 of the “V” prism 202.

In various preferred embodiments, the three optical fiber lines 206comprise fiber bundles such as incoherent fiber bundles. FIG. 11schematically illustrates one optical bundle 222 abutted to one lightsource 224 or light emitter so as to receive light from the lightsource. The optical fiber bundle 222 is split into two sections 226, 228that follow paths to two opposite ends of an optical device 230 such asa “V” prism. These two sections 226, 228 correspond to the first andsecond fiber lines 206 a, 206 b depicted in FIG. 10.

The fiber bundles 222 preferably comprise a plurality of optical fibers.The fiber bundles 222 may be split, for example, by separating theoptical fibers in the bundle into two groups, one group for the firstfiber line 206 a to the first input port 208 and one group for thesecond fiber line 206 b to the second input port 210. In variouspreferred embodiments, a first random selection of fibers is used as thefirst fiber line 206 a and a second random selection of fibers is usedas the second fiber line 206 b. To provide an approximately equaldistribution of light into the separate first and second lines 206 a,206 b directed to the first and second input ports 208, 210, the numberof fibers is preferably substantially the same in both the separatefirst and second lines 206 a, 206 b. This distribution can be adjustedby removing fibers from either the first or second of the fiber lines206 a, 206 b. Scaling, introducing correction with the spatial lightmodulator 236, can also be employed to accommodate for differences inthe illumination directed onto different portions of the display.

In one preferred embodiment, light emitted by the red, green, and bluelight sources 212 is introduced into the optical fiber bundle 222. Asdescribed above, this fiber bundle 222 is split such that the red light,the green light, and the blue light is input into opposite sides of the“V” prism 202. As is well known, light that appears white can beproduced by the combination of red, green, and blue. In addition, a widerange of colors can be produced by varying the levels of the red, green,and blue hues. Although three light sources 212 are shown comprisingred, green, and blue LEDs, more or fewer different colored light sourcesmay be provided. For example, four colored emitters may be employed thatinclude near blue and deep blue emitters for obtaining high colortemperature. Still more colors can be employed. In some embodimentseight or more colors may be included. Light sources other than LEDs mayalso be employed, and color combinations other than red, green and bluemay be used. Fluorescent and incandescent lamps (light bulbs) and laserdiodes are examples of alternative types of light sources. Other typesof sources are possible as well. Other color combinations include cyan,magenta, and yellow, although the specific colors employed should not belimited to those described herein. Various preferred embodiments includea plurality different color emitters that provide color temperaturesbetween about 3000 K and 8500 K (white), although this range should notbe construed as limiting.

Although the fiber bundle 222 is shown in FIG. 10 as being split intotwo portions 226, 228 corresponding to the two input ports 208, 210 ofthe “V” prism 202, the fiber bundle may be split further, for example,when the number of input ports is larger. In various embodiments,separate fiber bundles may be brought together at the source.Alternatively, a plurality of fiber bundles, one for each input port,may be positioned to couple light into the respective input port. Thesefiber bundles may be split into a plurality of ends that are opticallycoupled to the plurality of light emitters. Accordingly, light from thedifferent color emitters is brought together and input into the twosides of the prism 202. Various other combinations are possible.

In certain other embodiments, more than one set of emitters may beemployed, e.g., one set for each port 208, 210. Separate sources withseparate fiber bundles can be employed for separate ports 208, 210.Utilizing a common light source such as a common red, green, or blue LEDor LED array for the plurality of input ports, however, has theadvantage of providing uniformity in optical characteristics such as forexample in the wavelength of the light. Both sides of the “V” prism willthus preferably possess the same color.

A homogenizer such as an integrating rod, another form of light pipe,may also be employed to mix the red, green, and blue light. Light boxessuch as cavities formed by diffusely reflecting sidewalls may be used aswell for mixing and/or for conveying light. A fiber bundle can beoptically connected to a light pipe such as a conduit or a single large(or smaller) core fiber. In other embodiments, the fiber bundle can bealtogether replaced with optical fiber or flexible or rigid light pipes,or optical couplers, which may have large core or small core. Variouscombinations, e.g., of light sources, light piping, optical fiber andoptical fiber bundles, and/or mixing components, etc., may also beutilized.

In certain preferred embodiments, individual red, blue, and greenconveyances from respective red, blue, and green emitters may be coupledto a mixing component such as a mixing rod or light box or other lightpipe where the different colors are combined. In other embodiments,light piping such as molded walls that form optical conduits may includea LED receiver cup for coupling from different color emitters, e.g.,red, green, and blue LEDs, through the light piping to a mixing areasuch as a light box that may be output to a lens or other opticalelement. Alternative configurations and combinations are possible andthe particular design should not be limited to those examplesspecifically recited herein.

To produce color images using the spatial light modulator, the differentcolor emitters can be time division multiplexed with each color emitterseparately activated for a given time thereby repetitively cyclingthrough the different colors. The spatial light modulator is preferablysynchronized with the cycling of the color emitters and can be driven toproduce particular spatial patterns for each of the colors. Atsufficiently high frequencies, the viewer will perceive a singlecomposite colored image. In other embodiments more fully describedbelow, the three colors can be separated out by color selective filtersand directed to three separate modulators dedicated to each of the threecolors. After passing through the respective spatial light modulators,the three colors can be combined to produce the composite color image.Exemplary devices for accomplishing color multiplexing include the“X-cube” or the “Philips prism”. In other embodiments, more colors canbe accommodated, e.g., with time division multiplexing and/or withadditional spatial light modulators.

As shown in FIG. 10, beam shaping optics 232 are disposed in an opticalpath between the optical fiber lines 206 a and the first input face 234of the “V” prism. These beam shaping optics 232 may comprise, forexample, a refractive lens element or a plurality of refractive lenselements. Alternatively, diffractive optical elements, mirrors orreflectors, graded index lenses, or other optical elements may also beemployed. In various preferred embodiments, the beam shaping optics 232has different optical power for different, e.g., orthogonal directions.The beam shaping optics, 232, may for example, be anamorphic. The beamshaping optics 232 preferably has different optical power for orthogonalmeridianal planes that contain the optical axis through the beam shapingoptics 232. For example, the beam shaping optics 232 may comprise ananamorphic lens or anamorphic optical surface. A cylindrical lens may besuitably employed in certain preferred embodiments. In one preferredembodiment, the beam shaping optics 232 comprises a lens having anaspheric surface on one side and a cylindrical surface on another side.The cylindrical surface has larger curvature in one plane through theoptical axis and smaller or negligible curvature in another planethrough the optical axis. Preferably, the beam shaping optics 232 isconfigured to produce a beam or illumination pattern that is asymmetric.The beam may, for example, be elliptical or otherwise elongated,possibly being substantially rectangular, so as to illuminate arectangular field. The rays of light corresponding to the beam exitingthe beam shaping optics 232 may be bent (e.g. refracted) more in onedirection than in another orthogonal direction. Accordingly, thecorresponding rays of light may diverge at wider angles, for example, inthe X-Y plane than in the Y-Z plane. In some embodiments, integratingrods having rectangular cross-section or a fly's eye lens withrectangular lenslets may illuminate a rectangular field. Othercross-sections and shapes may be used to illuminate areas other thanrectangular. Although the beamshaping optics 232 is described aspreferably being anamorphic or have different optical power in differentdirections, in some embodiments, the beam shaping optics need not be soconfigured.

The beam shaping optics 232 also may be configured to provide asubstantially uniform distribution of light over the desired field. Thisfield may correspond, for example, to the reflective surface of the “V”prism 202 or the corresponding portion of a LCD array 236 disposed withrespect to an output of the “V” prism to receive light therefrom. Theluminance may be substantially constant across the portion on the LCD236 to be illuminated. In certain embodiments, preferably substantiallyuniform luminance is provided across the pupil of the optical system.This pupil may be produced by imaging optics, e.g., in the head-mounteddisplay or other projection or display device. Control over the lightdistribution at the desired portion of the spatial light modulator 236may be provided by the beamshaping optics 232.

The optical system 200 further comprises a collimating element 238 whichpreferably collimates the beam as shown in FIG. 10. The collimatingelement 238 depicted in FIG. 10 comprises a Fresnel lens, whichadvantageously has reduced thickness and is light and compact. Othertypes of collimating elements 238 may also be employed, such as otherdiffractive optical elements, mirrors, as well as refractive lenses. Forexample, the Fresnel lens could be replaced with an asphere, however,the Fresnel lens is likely to weigh less. In the embodiment illustratedin FIG. 10, the Fresnel lens is proximal the input face 234 of the “V”prism 202. As described above, in the case where the beamshaping optics232 is configured to produce a uniform light distribution, theilluminance at the collimating element 238 preferably is substantiallyconstant. The collimating lens 238 may also be anamorphic to collimatean elliptical or elongated beam.

An optical diffuser 240 is also disposed in the optical path of the beamto scatter and diffuse the light. In various preferred embodiments, thediffuser 240 spreads the light over a desired pupil such as an exitpupil of the imaging or projection optics 54 (see FIGS. 3 and 4). Thediffuser 240 is also preferably configured to assist in filling thepupil. The pupil shape is the convolution of the diffuser scatterdistribution and the angular distribution exiting the Fresnelcollimating element 238.

In some embodiments, the diffuser 240 also preferably assists inproviding a uniform light distribution across the pupil. For example,the diffuser may reduce underfilling of the pupil, which may cause thedisplay to appear splotchy or cause other effects. As describe morefully below, when the viewer moves his/her eye around, the viewer wouldsee different amounts of light at each eye position. In variousembodiments, for example, the f-number of the cone of rays collected bythe projection optics or imaging optics varies with position (e.g.,position on the spatial light modulator). Underfilling for somepositions in the spatial light modulator causes different levels offilling of the imaging optics pupil for different field positions, whichproduces variations observed by the viewer when the eye pupil moves.Uniformity is thereby reduced. Preferably the imaging system pupil isnot underfilled. Conversely, if the pupil is overfilled, light iswasted. The Fresnel lens also preferably avoids overfilling andinefficient loss of light. Accordingly, diffuser designs may be providedfor tailoring the fill, such that the pupil is not overfilled. Thecollimating lens used in combination with the diffuser aids incountering underfilling.

A variety of types of diffusers such as for example holographicdiffusers may be employed although the diffuser should not be limited toany particular kind or type. The diffuser 238 may have surface featuresthat scatter light incident thereon. In other embodiments, the diffusersmay have refractive index features that scatter light. Different designsmay be used as well. A lens array such as one or more fly's eye lensescomprising a plurality of lenslets can also be used. In such a case, thelenslets preferably have an aspheric surface (e.g., a conic profile or acurve defined non-zero conic constant) suitable for fast optical systemssuch as about f/1.3 or faster.

The diffuser 238 may also be combined with a polarizer or the Fresnellens or the polarizer and/or the Fresnel lens may be separate from thediffuser. Preferably, however, the polarizer is included in the opticalpath of the beam before the reflective beamsplitting surface of thebeamsplitter 202. Accordingly, this polarizer is referred to herein asthe pre-polarizer. Different types of polarizers that providepolarization selection may be employed including polarizers thatseparate polarization by transmitting, reflecting, or attenuatingcertain polarizations depending on the polarization. For example,polarizers that transmit a first polarization state and attenuate asecond polarization state, polarizers that transmit a first polarizationstate and reflect a second polarization state, and polarizers thatreflect a first polarization state and attenuate a second polarizationstate may be employed. Other types of polarizers and polarizationselective-devices may be employed as well.

The pre-polarizer is preferably oriented and configured such that lightpropagating therethrough has a polarization that is reflected by thepolarization beamsplitting surface in the prism 202. Preferably,substantially all of the light entering the input port 208, 210 ispolarized so as to be reflected by the polarization beamsplittingsurface and thereby to avoid transmission of light through thepolarization beamsplitting surface. If such light leaks through, e.g.,the first polarization beamsplitting surface and reaches the secondreflective surface, this light may be reflected by the second surfaceand may continue onto the output. Such leakage may potentially wash outthe pattern produced by the LCD and/or create imbalance between twosides of the output. A post-polarizer 241 disposed at the output of theV-prism may reduce this effect by removing the polarization that leaksthrough the first polarization beamsplitting surface and is reflected bythe second polarization beamsplitting surface in the V-prism 202.Accordingly, this post-polarizer 241 preferably removes light having apolarization that is selected to be reflected by the first and secondpolarization beamsplitting surfaces within the V prism 202. Both thepre-polarizers and the post-polarizer 241 may comprise polarizerscurrently known as well as polarizers yet to be devised. Examples ofpolarizers include birefringent polarizers, wire grid polarizers, aswell as photonic crystal polarizers.

The optical apparatus 200 depicted in FIG. 10 includes beamshapingoptics 232, collimating elements 238, diffusers 240, and polarizers foreach port. Accordingly, for the “V” prism 202 having two ports 208, 210,a pair of each of these components is shown. In other embodimentscomprising more ports, the additional input ports may be similarlyoutfitted with beamshaping optics, collimating elements, diffusers, andpolarizer's. Other elements such as filters etc. can also be includedand any of the elements shown may be excluded as well dependingpotentially on the application or design. Various other combinations andarrangements of such elements are also possible.

As discussed above, light from the array of light sources 204 is coupledinto the optical fiber line 206 and distributed to the input ports 208,210 of the prism 202. The light output from the optical fiber 206 isreceived by the beamshaping optics 232, which preferably tailors thebeam substantially to the size and shape of the portion of the spatiallight modulator 236 to be illuminated. Similarly, the size and shape ofthe beam substantially may match that of an aperture or pupil associatedwith the optical system 200 in various preferred embodiments. The beammay be for example between about 5 and 19 millimeters wide along onedirection and between about 10 and 25 millimeters along anotherdirection. In various embodiments, the beamshaping optics 232 converts acircular shaped beam emanating from the optical fiber 206 a, 206 b intoan elliptical beam. The cross-section of the beam exiting the opticalfiber 206 taken perpendicular to the direction of propagation of thebeam is generally circular. The beam shaping optics 232 preferably bendsthe beam accordingly to produce a perpendicular cross-section that isgenerally elliptical or elongated. This shape may be substantiallyrectangular in some embodiments.

Preferably, the beamshaping optics 232 also provides for more uniformdistribution across the spatial light modulator 200. The beam exitingthe optical fiber 206 may possess a substantially Gaussian intensitydistribution with falloff in a radial direction conforming approximatelyto a Gaussian function. Such a Gaussian intensity distribution mayresult in a noticeable fall off in light at the LCD 236. Accordingly,the beamshaping optics 232 preferably produces a different distributionat the LCD 236. In certain preferred embodiments discussed more fullybelow, the beamshaping optics 232 is configured such that the light atthe LCD 236 has a “top hat” or “flat top” illuminance distribution whichis substantially constant over a large central region.

FIGS. 12 and 13 show the illuminance distribution at the spatial lightmodulator 234 for the respective first and second input ports 208, 210of the “V” prism 202. This illuminance distribution is substantiallyGaussian. A cross-section of a Gaussian illuminance distribution such asacross the line 14-14 in FIG. 12 is presented in FIG. 14. The Gaussianhas a peak with an apex and sloping sides. As shown, this Gaussian iscircularly symmetric about the Z-axis. FIGS. 12 and 13 also show aportion of the perimeter 242 of the output face. One edge 244 of theperimeter corresponds to the vertex of the prism. For each side, a peakis centrally located within the rectangular field of the reflectivesurface of the prism and/or the rectangular portion of the LCD 236.

FIG. 15 schematically illustrates flux from the two sides of the “V”prism 202 combined together for example at the spatial modulator 236.FIG. 15 also shows a perimeter 242 corresponding to the two portions ofthe output face associated with the two sides of the “V” prism 202,respectively. This perimeter may likewise correspond to the two portionsof the spatial light modulator 236. Two peaks in the illuminancedistribution are centrally located within each of the rectangularportions of the LCD 236.

The light beam may be offset such that the peak is shifted from centerin one direction as illustrated in FIGS. 16 and 17, which show theilluminance incident on the two portions of the spatial light modulatorcorresponding to the two sides of the “V” prism. FIGS. 16 and 17 show aperimeter 242 delineating the two portions of the spatial lightmodulator 234 coinciding with the reflective surfaces in the prism 202,and/or the output faces. Line 244 on the perimeter 242 corresponds tothe vertex of the prism. The illuminance is represented as a Gaussiandistribution with a peak shifted in the Y direction from the center ofthe spatial light modulator 234. The light beam may be shifted oraltered in other ways to preferably provide more uniform illumination.

In various exemplary embodiments that employ Koehler illumination, thefalloff in the source angular distribution maps to the corners of thetwo output portions of the “V” prism 202 as well as, for example, to thecorresponding portions of the spatial light modulator 236. (In Koehlerillumination, the light source is imaged in the pupil of the projectionoptics, e.g., at infinity.) If the falloff is sufficiently slow and nottoo large, the observable variation in light level may not besignificant. If however, the falloff is sharp and sizeable, thevariation across the output of the “V” prism 202 may result for examplein noticeable fluctuations in light reaching the eye in specificcircumstances.

In various embodiments, the illumination output by the prism 202,however, is preferably substantially constant and uniform. As discussedabove, therefore, a “top hat” or “flat top” illuminance distribution maybe preferred over the Gaussian distribution. A substantially “top hat”illuminance distribution incident on the output face 234 of the prism202 is shown in FIG. 18. A cross-section of the “top hat” illuminancedistribution across the line 19-19 is presented in FIG. 19. The “tophat” distribution is substantially constant over a central portion 246and falls off rapidly beyond the substantially constant central portion.The width of the substantially constant central portion 246 ispreferably sufficiently large so as to fill the appropriate area, suchas for example the eye pupil in certain display embodiments such as forhead mounted and helmet mounted displays. In the case where the “tophat” distribution is substantially constant within the central portion246, substantially constant illuminance across the pupil may beprovided. This “top hat” distribution is shown as circularly symmetricabout the Z-axis although asymmetric such as elliptical shapes may bepreferred. FIG. 18 also shows the perimeter 242 of the portion of thespatial light modulator 234 illuminated by one side of the V-prism, orthe corresponding reflective surface and/or output face of the prism202. One edge 244 of the perimeter 242 corresponds to the vertex of theprism 202. Although a “top hat” distribution is shown, otherdistributions wherein the light level, e.g., illuminance, issubstantially constant may be employed. Preferably, the illuminance issubstantially constant at least across a portion of the “V” prism 202output corresponding to the relevant pupil such as the pupil of the eyefor certain embodiments.

The intensity exiting the optical fiber 206 a, 206 b may be moreGaussian than “top hat” or “flat top” resulting in more falloff. Asdiscussed above, clipping the rotationally symmetric angulardistribution with a rectangular field can produce more significantfalloff near the center of the spatial light modulator 236 andconsequently at the center of the display or projection screen since thevertex of the “V” prism 202 corresponds to the center of the output ofthe “V” prism. In certain embodiments, therefore, the beamshaping optics232 preferably provides a substantially “top hat” illuminancedistribution at the spatial light modulator 234. A lens 232 that isaspheric at least on one of the optical surfaces may yield such adistribution. An integrating rod may also output a substantiallyconstant illumination distribution like a flat top distribution thatfalls of rapidly. When using an integrating rod or light pipe thatprovides substantially constant illumination beam shaping optics may ormay not be used to further flatten the illumination distribution. (Invarious embodiments, preferably the diffusers as well as the collimatormay be employed with the integrating rod or light pipe, e.g., toincrease uniformity. The diffuser may, for example, be used instead oflonger integrating rods or light pipes, thereby increasing compactness.)

Asymmetric beamshaping optics 232 are also preferably used to produce anasymmetric beam. For example, a cylindrical lens having a cylindricalsurface may advantageously convert the circular peaked distribution intoa distribution having a central oval portion, more suitable for therectangular field. As described above, the beamshaping optics 232 maycomprise one or more refractive elements having an aspheric surface andan anamorphic (e.g., cylindrical) surface. As stated above, anintegrating rod having an asymmetric (e.g., rectangular) cross-sectionor a fly's eye lens comprising a plurality of asymmetrically shaped(e.g., rectangular) lenslets may be used to provide such asymmetric beampatterns. Other approaches to providing asymmetric distributions arepossible.

As will be discussed more fully below, the diffuser 240 is alsopreferably configured to provide substantially uniform light levels. Thediffuser may include a plurality of scatter features that scatterincident light into a cone of angles such as illustrated in FIG. 20. Thediffuser may be designed to substantially limit this cone of angles, θ.In addition, the diffuser may be configured to provide a specificangular distribution wherein the intensity varies with angle accordingto a distribution, I(θ). In certain preferred embodiments, for example,this angular distribution also substantially conforms to a “top hat”distribution. Top hat and Gaussian angular distributions 248, 250 areplotted in FIG. 21. (Such distributions are similar to correspondingBidirectional Scatter Distribution Functions, BSDFs). For the Gaussiandistribution 250, the intensity peaks for a central angle, θ_(o), butfalls off gradually for angles larger and smaller than the centralangle. In contrast, for the “top hat” distribution 248, a portion 252 ofthe angles have a substantially similar intensity level. For anglesoutside that region 252, however, the intensity rapidly drops off. Sucha distribution 248 may be useful for efficiently distributing the lightto the desired areas without unnecessary and wasteful overfill.

The size of the spatial light modulator 236 may be between about 6 to 40millimeters or between about 12 to 25 millimeters on a diagonal. Incertain embodiments, the spatial light modulator 236 may have shapesother than square, and may for example be rectangular. In one exemplaryembodiment, the aspect ratio of the spatial light modulator that isilluminated is about 3:4. Dividing the illuminated region in two mayyield an aspect ratio of about 3:8 for the section of the spatial lightmodulator illuminated by one side of the V-prism. More broadly, theportion illuminated by one half of the output port may be between about2×4 millimeters to 14×28 millimeters, although sizes outside theseranges are possible. Still other shapes, e.g., triangular, are possible.Accordingly, the beam used to illuminate the spatial light modulator 236may have a length and width between about 2×4 millimeters to 14×28millimeters, respectively. The collimator aperture, diffuser aperture,polarizer aperture as well as the input faces 234 and reflectivesurfaces of the prism 202 may have aperture sizes in one directionbetween about 2 and 14 millimeters and in another direction betweenabout 4 and 28 millimeters. The dimensions, however, should not belimited to those recited here.

FIG. 22 depicts a field-of-view 265 for a display such as a head mounteddisplay produced by a V-prism. A dark stripe 266 is visible at thecenter of the field 265. This stripe 266 results from the finitethickness of the beamsplitting reflective surfaces 268 of the V prism,which is shown in FIG. 23. In the case where polarization beam splittingis provided by a plurality of birefringent layers, the stack ofbirefringent layers introduces this thickness. In the case where thepolarization beam splitting layer comprises a wire grid, the height ofthe wires contributes to this thickness. Other structures such asphotonic crystal polarizers have finite thickness, which may cause thisstripe to be visible. A portion 270 of the output of the V-prism, isaffected by the reduced performance of the beamsplitting surfaces. Thisregion 270, as well as the thickness of the beamsplitting layers 268,has been exaggerated in this schematic drawing and, accordingly, is notto scale. The stripe 266 shown in FIG. 22 is likewise exaggerated aswell and is preferably not visible to the viewer.

To decrease the size of the stripe 266, the thickness of thepolarization beamsplitting layer 268 is preferably reduced. Preferably,the thickness is not larger than a few percent of the beam at the pupilof the system. In various preferred embodiments, for example, thethickness of the polarization beamsplitting structure 268, e.g., thethickness of the multiple birefringent layer stack or the photoniccrystal polarizers is less than about 5 to 100 micrometers. Thicknessesoutside this range, however, are possible. A post-polarizer 272 may alsobe included to potentially reduce this effect.

FIGS. 24-26 depict cross-sectional views of wire grid polarizers 275.The wire grid polarizer 275 comprises a plurality of elongated strips276 preferably comprising metal such as aluminum. The elongated strips276 are arranged parallel to each other. The height of the wires 276 isbetween about 20 to 60 nanometers, although larger or smaller strips maybe employed in different embodiments. The strips 276 may have a width ofbetween about 10 and 90 nanometers and a periodicity of between about 50and 150 nanometers. The strips 276 may be separated by a distance toprovide a duty cycle of between about 0.25 and 0.75. The periodicity ispreferably sufficiently small for the wavelengths of use such that theplurality of strips 276 does not diffract light into different orders.Light will therefore be substantially limited to the central or zeroorder. Values outside these ranges, however, are possible.

In FIG. 24, the strips 276, formed on a substrate 278, are separated byopen spaces such as air gaps 280. A layer of glue 282 or other adhesivematerial is employed to affix a superstrate 284 to the wire gridpolarizer. In such an embodiment, preferably the glue 282 is viscous anddoes not fill in the open regions 280 separating the strips 276. In FIG.25, glue 282 fills these open regions 280. In various preferredembodiments, the glue 282 has an index of refraction similar to that ofthe substrate 278 and/or superstrate 284. Accordingly, if the substrate278 and superstrate 284 comprise BK7 glass, preferably the glue 282 hasan index of refraction of about 1.57. FIG. 25 shows a layer of oxide 286such as aluminum oxide (Al₂O₃) that may be formed on metal strips 276comprising for example aluminum. FIG. 26 shows a layer of MgF 288 formedover the array of strips. This layer of MgF may range between about 0.5and 20 microns thick although other thicknesses outside this range arepossible. The MgF is shown in the regions separating the strips 276 aswell in this exemplary embodiment. Other materials beside MgF, such as,for example, silica may be employed in other embodiments of theinvention.

One exemplary process for forming the wire grid polarizers 275 in theV-prism is illustrated in FIGS. 27A-27G. Preferably, substantiallysmooth surfaces 502 are formed on a first triangular prism 504, forexample, by polishing as shown in FIG. 27A. This prism 504 may compriseglass such as BK7 or SF57 or other glass or substantially opticallytransmissive material. In certain preferred embodiments, this prism 504has a cross-section in the shape of a right triangle having a hypotenuse506. The surfaces 502 of this prism are preferably substantially planar,at least those corresponding to the hypotenuse 506 and one of the sidesopposite the hypotenuse shown in the cross-section.

A first wire grid polarizer 508 is formed on a side of the prism 504 asillustrated in FIG. 27B. Metal deposition and patterning may be employedto create an array of parallel metal strips comprising the wire gridpolarizer 508. These strips are shown as being formed on the surface 502corresponding to the hypotenuse 506 in the cross-section shown in FIG.27B. In certain preferred embodiments, the metal strips may be formed ona glass wafer 510 using lithographic processes. The wafer 510 may bediced into pieces that are bonded or adhered to the prism 504. Openspaces may separate the strips. An overcoat layer comprising, e.g., MgFor silica or other material, may be formed over the plurality of strips.

A second triangular prism 514 similar to the first triangular prism 504is attached to the first triangular prism sandwiching the first wiregrid polarizer 508 between the two prisms as depicted in FIG. 27C. Thissecond prism 514 may also comprise glass such as BK7 or SF57 or otherglass or substantially optically transmissive material. Similarly, thissecond prism 514 may have a cross-section in the shape of a righttriangle having a hypotenuse 516. At least the surface corresponding ofthe hypotenuse 516 shown in the cross-section is preferablysubstantially planar. A substantially cylindrical structure having asubstantially square cross-section is formed by attaching the secondtriangular prism 514 to the first triangular prism 504.

The first and second triangular prisms 504, 514 together with the firstwire grid polarizer 508 sandwiched therebetween are cut and/or polishedalong a diagonal of the square cross-section formed by attaching thefirst triangular prism to the second triangular prism as shown in FIG.27D. A substantially cylindrical structure 524 having a substantiallytriangular cross-section is thereby created. This triangularcross-section 524 is a right triangle with a hypotenuse 526 that ispreferably substantially orthogonal to the first wire grid polarizer508.

A second wire grid polarizer 538 is added to the substantiallytriangular cylindrical structure 524 as shown in FIG. 27E. The secondwire grid polarizer 538 may be created by depositing and patterningmetal to form a plurality of parallel metal strips. As described above,in certain preferred embodiments, the metal strips may be formed on aglass wafer 540 using lithographic processes. The wafer 540 may be dicedinto pieces that are bonded or adhered to the prism 504. An overcoatlayer comprising, e.g., MgF or silica or other material, may be formedon the second wire grid 538. As illustrated in FIG. 27E, the second wiregrid polarizer 538 is disposed on a surface of the substantiallycylindrical structure 524 corresponding to the hypotenuse 526 of thetriangular cross-section. Accordingly, the second wire grid polarizer538 is preferably approximately orthogonal to the first wire gridpolarizer 508.

A third triangular prism 534 similar to the first and second triangularprisms 504, 514 is attached to the first and second triangular prismssandwiching the second wire grid polarizer 538 therebetween (see FIG.27F). This third prism 534 may also comprise glass such as BK7 or SF57or other glass or substantially optically transmissive material.Similarly, this third prism 534 may have a cross-section substantiallyin the shape of a right triangle having a hypotenuse 536, and at leastthe surface of this third triangular prism 534 corresponding to thehypotenuse is preferably substantially planar. The surface correspondingto the hypotenuse 536 of the third triangular prism is preferablyadjacent to the second wire grid 538 or the overcoat layer formedthereon. A substantially cylindrical structure 544 having asubstantially square cross-section is thereby formed by attaching thethird triangular prism 534 to the first and second triangular prisms504, 514. This square cross-section has four sides, two sides areprovided by the first and second triangular prisms 504, 514respectively, and two sides are provided by the third triangular prism534. The first wire grid 508 partly extends along a portion of a firstdiagonal of this square cross-section while the second wire grid 538extends along a second diagonal of the square cross-section that isorthogonal to the first diagonal.

The first, second, and third triangular prisms 504, 514, 534 togetherwith the second wire grid polarizer 538 are cut and/or polished therebyremoving portions of the third triangular prism and portions of eitherthe first or second triangular prisms along one side of the generallysquare cross-section. In FIG. 27G, portions of the first triangularprism 504 are removed together with portions of the third triangularprism 534. In certain preferred embodiments, a substantially planarsurface 542 is formed by cutting and/or polishing. Preferably, theportions of the first and second wire grid 508, 538 that remain extendtoward this substantially planar surface 542 at an angle of about 40° to50° to this substantially planar surface, and about 80-100° with respectto each other. Additionally, sufficient material is removed by cuttingand/or polishing such that the portions of the first and second wiregrid 508, 538 also preferably extend to this substantially planarsurface 542. The result is a V-prism 550. In the case where MgF coatingsare employed, a slight asymmetry may be introduced depending on whethermaterial is removed by polishing the first or second triangular prism504, 514 together with the third triangular prism 534.

Variations in the process of forming the V-prism are possible. Forexample, substantially planar surfaces need not be formed in certainembodiments. Curved surfaces on the V-prism that have power may beformed. Different methods of fabricating the wire grid polarizers 510,538 are also possible and one or both of the MgF layers 510, 540 may ormay not be included. Additional processing steps may be added or certainsteps may be removed, altered, or implemented in a different order. Incertain embodiments, for example, a flat with a wire grid formed thereonmay be cemented to the triangular prism instead of depositing andpatterning the plurality of metal strips directly on the prism. Othertechniques for forming the V-prism including those yet devised may beemployed as well.

In various preferred embodiments, the optical system 200 may furthercomprise an optical wedge 254 with the V-prism. This optical wedge 254may for example be disposed between the (intermediate) output face ofthe “V” prism and the spatial light modulator 236 as shown in FIG. 28.The wedge 254 may comprise, for instance, a plate of material such asglass that is substantially optically transmissive to the light. Theplate, however, has one surface tilted with respect to the other. Thethickness of the wedge 254, therefore, varies across the field. Theoptical wedge 254 introduces astigmatism and coma when the beam isfocused through the wedge. This astigmatism and coma can be employed tooffset astigmatism and coma introduced by other optical elements such asthe imaging optics 54. Optical wedges are described, for example, inU.S. Pat. No. 5,499,139 issued to Chen which is hereby incorporatedherein by reference in its entirety.

The optical wedge 254 shown in FIG. 28 is separated from the prism 202by a gap, which may be an air gap. In contrast, FIG. 29 shows awedge-shaped prism 256 wherein the wedge is incorporated in the prism.The wedge-shaped prism 256 may for example have one output surface, theintermediate output, tilted with respect to the other output surface.This prism 256 also introduces astigmatism and coma and can be used tocounter these effects introduced by components elsewhere in the system200. In certain circumstances, however, the wedge 254 separated from theprism 202 by a gap yields improved optical performance.

In certain embodiments wherein the wedge-shaped prism 256 is employed,the diffuser preferably has a “top hat” angular distribution 248 such asshown in FIG. 21, which provides increased uniformity. Otherwise, theilluminance distribution may exhibit additional non-uniformities. FIG.30 shows a plot of illuminance across the liquid crystal spatial lightmodulator 236 for embodiments that include a wedge-shaped prism 256. Adiffuser 240 having a Gaussian angular distribution 250 such as shown inFIG. 21 yields an illuminance distribution shown by a first plot 258that has a dip in the illuminance. A diffuser 240 having a “top hat”angular distribution 248 such as shown in FIG. 21 yields an illuminancedistribution shown by a second plot 260 having a substantially constantilluminance across the field. The wedge-shaped prism 256 can be replacedwith a prism 202 and wedge 254 combination such as shown in FIG. 28wherein a gap separates the prism and the spatial light modulator 236. Asubstantially constant illuminance results. Such a configuration willalso reduce angular uniformity requirements of the diffuser 240. Forexample, both diffusers 240 with Gaussian distributions and diffuserswith “flat top” distributions can perform suitably well.

A mapping of the illuminance across the spatial light modulator 236 fora wedge-shaped prism 256 having a 1.3° wedge is shown in FIG. 31.Substantial uniformity is demonstrated. FIG. 32 is a histogram of theluminous flux per area (in lux). This plot shows that the luminous fluxper area received over the spatial light modulator 236 is within anarrow range of values.

The uniformity is greater for the example wherein the wedge 254 isseparate from the prism 202 with an air gap therebetween. FIG. 33 showsa mapping of the illuminance for such a case. The variation is within±12%. FIG. 34 shows the smaller range of variation in illuminance level.The illuminance level may depend on the particular system design orapplication. Values outside these ranges are possible as well.

The wedge-shaped prism 356 may also demonstrate improved performance ifthe “V” is rotated with respect to the tilted surface forming the wedge.In such a configuration, the thickness of the wedge increases (ordecreases) with position along a direction parallel to the edge thatforms the apex of the “V” shaped component.

A color splitting prism may also be included together with the V-prismin certain embodiments to provide color images, graphics, text, etc.FIG. 35 illustrates an optical system 600 for a projector comprising aV-prism 602 and an X-cube 604. The V-prism 602 is disposed between aprojection lens 606 and the X-cube 604. X-cubes are available from 3M,St. Paul, Minn.

The V-prism 602 comprises first and second input ports 608 for receivingillumination that is preferably polarized. The V-prism 602 furthercomprises first and second polarization beamsplitting surfaces 610 forreflecting the illumination received through the first and second inputports 608. The first and second polarization beamsplitting surfaces 610are oriented to reflect light received through said first and secondinput ports 608 to a central input/output port 612 of the X-cube 604.

The X-cube 604 additionally comprises first and second reflective colorfilters 614 that reflect certain wavelengths and transmit otherwavelengths. The first and second reflective color filters 614preferably have respective wavelength characteristics and are disposedaccordingly to reflect light of certain color to first and second colorports 616 where first and second spatial light modulators 618 arerespectively disposed. The X-cube 604 further comprises a third colorport 620 located beyond the first and second reflective color filters614 to receive light not reflected by the first and second reflectivecolor filters. A third spatial light modulator 622 is disposed toreceive light from this third color port 620. In various preferredembodiments, reflective spatial light modulators that selectivelyreflect light may be employed to create two-dimensional spatialpatterns. Light reflected from the first and second spatial lightmodulator 618 through the respective port 616 will be reflected from thefirst and second reflective color filters 614 respectively. Lightreflected from the third spatial light modulator 622 through the thirdcolor port 620 will be transmitted through the first and secondreflective color filters 614. The light returned by the spatial lightmodulators 618, 622 will therefore pass through the X-cube 604 and thecentral input/output port 612 of the X-cube. This light will continuethrough the V-prism 602 onto and through the projection optics 606 to ascreen 624 where a composite color image is formed for viewing.

Other components, such as e.g., polarizers, diffusers, beamshapingoptics etc., may also be included. Optical wedges may be included aswell between the X-cube 604 and the spatial light modulators 618, 622 incertain embodiments. Other designs, configurations, and modes ofoperation are possible.

Other types of color devices may also be employed. FIG. 36 illustratesan optical system 650 for a rear projection television comprising aV-prism 652 and a Philips prism 654. The V-prism 652 is disposed betweena projection lens 656 and the Philips prism 654. Philips prisms areavailable from Richter Enterprises, Wayland, Mass.

The V-prism 652 comprises first and second input ports 658 for receivingillumination that is preferably polarized. The V-prism 652 furthercomprises first and second polarization beamsplitting surfaces 660 forreflecting the illumination received through the first and second inputports 658. The first and second polarization beamsplitting 660 surfacesare oriented to reflect light received through said first and secondinput ports 658 to a central input/output port 662 of the Philips prism.

The Philips prism 654 additionally comprises first and second reflectivecolor filters 664, 665 that reflect certain wavelengths and transmitother wavelengths. The first and second reflective color filters 664,665 preferably have respective wavelength characteristics and aredisposed accordingly to reflect light of certain color to first andsecond color ports 666, 667 where first and second spatial lightmodulators 668, 669 are respectively disposed. The Philips prism 654further comprises a third color port 670 located beyond the first andsecond reflective color filters 664, 665 to receive light not reflectedby the first and second reflective color filters. A third spatial lightmodulator 672 is disposed to receive light from this third color port670.

In various preferred embodiments, reflective spatial light modulatorsthat selectively reflect light may be employed to create two-dimensionalspatial patterns. Light reflected from the first and second spatiallight modulator 668, 669 through the respective port 666, 667 will bereflected from the first and second reflective color filters 664, 665,respectively. Light reflected from the third spatial light modulator 672through the third color port 670 will be transmitted through the firstand second reflective color filters 664, 665. The light returned by thespatial light modulators 667, 668, 672 will therefore pass through thePhilips prism 654 and the central input/output port 662 of the Philipsprism. This light will continue through the V-prism 652 onto and throughthe projection optics 656 to a pair of mirrors (not shown) for forming acomposite color image on a screen for viewing. As described above, othercomponents, such as, e.g., polarizers, diffusers, beamshaping optics,etc., may also be included. Additionally, optical wedges may be includedbetween the Philips prism 654 and the spatial light modulators 668, 669,672 in certain embodiments.

FIGS. 35 and 36 do not show the optical components used to couple lightto the V-prisms 602, 652. As discussed above, however, illumination maybe provided using light pipes and light boxes including conformal wallsthat define cavities for light to flow as well as mirrors and otherrefractive, reflective, and diffractive optical components. Illuminationmay also be provided by optical fibers, fiber bundles, rigid or flexiblewaveguides, etc. In certain cases where compactness is a consideration,such configurations may be designed to reduce overall size.

FIG. 37 shows one example where a mirror 270 for folding the input beammay be included in the optical system 200 to reduce the width of thesystem. The folding mirror 270 may comprise a planar specularlyreflective surface such as shown or may comprise other reflectiveoptical elements as well. As depicted in FIG. 37, this beam foldingmirror 270 may be easily integrated into the illuminator optical path.The fold mirror 270 bends the optical path of the beam reducing thewidth of the system, and thereby facilitating compact packaging. Invarious preferred embodiments, this optical path is bent by about 90°,however, different angles are possible as well. A dimension, d,corresponding to the width of the system is shown in FIG. 37. In variouspreferred embodiments, this dimension, d, may be 1-3 inches, andpreferably about 2 inches. Sizes outside this range are also possible.The folding mirror may, however, increase stray light effects.

In various embodiments, non-uniform controlled illumination at thespatial light modulator 236 is desired. For example, in some cases,uniform illuminance at the spatial light modulator 236 (with anintensity distribution that falls off only slightly towards higherangles) produces a non-uniform distribution at the output of the opticalsystem. As discussed above, in many optical imaging systems, forinstance, the f-number or cone of rays collected by the optical systemvaries across the field due to distortion. Uniformly illuminating theobject field of such an imaging system results in the collection ofdifferent amounts of light from different locations in the object fieldand corresponding illuminance variation at the image plane. Non-uniformillumination at the spatial light modulator, may compensate for thiseffect and provide uniformity at the image field.

Accordingly, if a uniform spatial illuminance distribution across thedisplay results in a gradation in the uniformity seen by the observer, anon-uniform illuminance can be used to compensate for the gradation. Onemethod for achieving a compensating linear variation in the illuminanceis to use an off-axis illumination such as shown in FIG. 38. In thisembodiment, the optical axis of the fiber output and the beamshaperoptics 236 is oriented at an oblique angle with respect to the opticalaxis through the Fresnel collimating lens 238, diffuser 240, polarizer,and the “V” prism input 234. The light source and beamshaping optics 236are appropriately rotated with respect to the Fresnel lens 238 and the“V” prism 202, and the Fresnel lens, diffuser 240, and “V” prism aredecentered with respect to the fiber output and the beam shaping optics232. Similarly, the optical path of the beam of light propagating fromthe fiber optic 206 a, 206 b and the beamshaping optics 232 to theFresnel collimating lens 238 is angled with respect to the optical pathof the beam through the Fresnel lens, diffuser 240, polarizer and input234 of the “V” prism 202. FIG. 38 depicts the rotation of thebeamshaping optics about an axis parallel to the Z axis and thedecentering in the X direction. This tilt of the light source withrespect to the V-prism may, for example, range between about 50 to 45°,e.g., about 26°. The decenter of the light source with respect to thecentral axis through the V-prism may be between about 11 and 25millimeter in some cases. Values outside these ranges, however, arepossible.

In this embodiment, the beamshaping optics 232 comprises a lens having acylindrical surface. As described above, this cylindrical surfaceimproves collection efficiency of the rectangular input face of the “V”prism 202. The resultant efficiency is substantially similar to theefficiency achieved in the uniform luminance configurations. Otherelements within the optical system 200 may be tilted, decentered and/oroff-axis as well. In addition, not all of the components need to betilted, decentered, and off-axis in every embodiment. Other variationsare possible.

The result of the tilt and decenter is that the illuminance across theFresnel collimating lens 238, diffuser 240, polarizer, input 234 of theprism 202, and liquid crystal spatial light modulator 236 isnon-uniform. In particular, in this embodiment, the illuminance acrossthe intermediate output of the “V” prism 202 and at the spatial lightmodulator 236 is graded as shown by the plots in FIGS. 39 and 40. Inthis embodiment, this gradation from high to low illuminance extendsalong the X direction parallel to the vertex of the “V” prism. As shown,the optical path distance from the beam shaping optics 232 to theFresnel collimating lens 238 varies across the field introducing acorresponding variation in the illuminance.

Preferably, the configuration is selected to provide the desiredillumination, which may be a specific illumination of the object fieldto counter non-uniformity in the optics, e.g., imaging optics 54, and toultimately yield uniformity in the image plane. One exemplaryconfiguration is the off-axis illumination depicted in FIG. 38, whichcan be suitably adjusted to offset non-uniformities in off-axis imagingsystems 54 and provide uniformity in the image field. Otherconfigurations, however, adjusted in a variety of ways may be utilizedto provide the desired effect. For example, an absorption plate havinggraded transmission properties or transmittance that varies withlocation along the width of the plate may be employed. Alternativedesigns are also possible. Also, although the illuminance is depicted asa generally decreasing value with position, X, along the width of thespatial light modulator 236, the variation in illumination can takeother forms. Preferably the system 200 is configured to provide thedesired illumination across the spatial light modulator 236. In somecases, the desired profile is a generally decreasing, e.g.,substantially monotonically decreasing illuminance across a substantialportion of the light spatial light modulator 236. For example, the ratioof illuminance from one end to another may range from about 2:1 to 6:1over a lateral distance of between about 15 to 45 millimeters. Thisdistance may be, for example, about 26 millimeters when the spatiallight modulator may be for example about 17×19 millimeters. Valuesoutside these ranges, however, are possible.

In various embodiments, the diffuser 240 is graded in the lateraldirection. The diffuser 240 includes a plurality of scattering (e.g.,diffractive features) laterally disposed at locations across thediffuser to scatter light passing through the diffuser. As shown inFIGS. 41 and 42, light incident on the diffuser is scattered by thesediffractive features into a plurality of directions filling a projectedsolid angle having a size determined by the diffractive features in thediffuser. As shown, the projected solid angle into which light isscattered may be different for different locations on the diffuser.Preferably, the scattering features in the diffuser are arranged suchthat the projected solid angle into which light is scattered increaseswith lateral position on the diffuser. Accordingly, light incident on afirst location 260 is scattered into a first projected solid angle Ω₁,light incident on a second location 262 is scattered into a secondprojected solid angle Ω₂, and light incident on a third location 264 isscattered into a third projected solid angle Ω₃. These locations areshown in FIG. 42 as being arranged sequentially along the X-direction.Similarly, the projected solid angle Ω₁, Ω₂, Ω₃ associated with thethree locations 260, 262, 264, progressively increases such that lightis dispersed into smaller angles for locations on one side of thediffuser and larger angles for locations on the other side of thediffuser.

Gradation in the scattering characteristics across the diffuser can beuseful in various applications. For example, as described above, theimaging optics may possess an f-number or numerical aperture andcorresponding collection angle that varies with field. If theillumination is reflected from the liquid crystal spatial lightmodulator 236 into a constant projected solid angle, the projected solidangle of the illumination may not match the respective collection angleof the imaging optics. The light from some field points on the liquidcrystal modulator 236 may fill the aperture of the imaging optics;however, the light from other field points may fail to fill thecorresponding aperture of the imaging optics.

For displays such as head-mounted including helmet-mounted displays, theaperture of the imaging optics preferably maps to the pupil of the eye12. If the aperture of the imaging optics is under-filled, slightmovement of the eye pupil may cause dramatic drop off in light receivedby the retina. Increased tolerance is therefore desirable as the eye andhead of the viewer may move laterally shifting the location of the eyepupil.

Overfilling is a possible solution. The projected solid angle into whichthe spatial light modulator emits light may fill the aperture of theimaging optics in each case, overfilling the aperture for some fieldpoints. This latter approach, however, is less efficient as lightoutside the aperture is discarded. Moreover, light that is outside theaperture of the imaging optics may not be absorbed and can scatter backinto the field-of-view, reducing the image contrast.

Accordingly, in various preferred embodiments, the projected solid angleinto which light propagates from the spatial light modulator 236 issubstantially matched to the corresponding collection angle of theimaging optics. For example, in cases where the f-number of the imagingoptics varies with field position, the projected solid angle associatedwith the output of the liquid crystal modulator 236 is preferablyfield-dependent as well. A graded diffuser such as described above canprovide this effect. The diffuser 240 preferably scatters light intoprojected solid angles that increase in size across the diffuser. Thislight illuminates the reflective spatial light modulator 236. The lightis reflected from the liquid crystal modulator 236 into projected solidangles that increase across the spatial light modulator. Preferably,these increasing projected solid angles substantially match thecollection angles of the imaging optics, which also increase with fieldposition. If the projected solid angles for the various points on thespatial light modulator 236 are substantially equivalent to therespective collection angles of the imaging optics, the aperture of theimaging optics will be efficiently filled for each particular fieldlocation.

In various preferred embodiments, non-uniform, and more specificallygraded illumination such as provided by the off-axis illuminationconfiguration shown in FIG. 38 is combined with a graded diffuser havingscatter properties that progressively vary with transverse locationacross the diffuser. Graded illuminance is illustrated in FIGS. 39 and40. Such an illuminance distribution across the diffuser can be pairedwith an increasingly large projected solid angle into which the diffuser240 scatters light. Preferably, this combination provides substantiallyconstant luminance as higher illuminance and wider projected solidangles can be selected to yield substantially the same luminance aslower illuminance and corresponding narrower projected solid angles.

In the example shown in FIG. 42, light incident on the first location260 has an illuminance I₁ and is scattered into the first projectedsolid angle Ω₁ to produce a resultant luminance L₁. Light incident onthe second location 262 has an illuminance I₂ and is scattered into thesecond projected solid angle Ω₂ to yield luminance L₂. Light incident onthe third location 264 has an illuminance I₃ and is scattered into thethird projected solid angle shown Ω₃ thereby providing a resultantluminance L₃. In this case, the illuminance increases progressively withlateral position across the diffuser 240 such that I₁<I₂<I₃. Similarly,the projected solid angle Ω₁, Ω₂, Ω₃ associated with the three locations260, 262, 264, is progressively wider. Accordingly, less light isdistributed over a smaller range of angles while more light isdistributed over a wider range of angles. In certain embodiments, forexample, the projected solid angle may range from 0 to π radians acrossthe diffuser. The ratio of projected solid angles from one end of thediffuser to another end of the diffuser used to illuminate the spatiallight modulator may range, for example, from 2:1 to 6:1. Values outsidethese ranges, however, are possible. Substantially constant luminanceacross the diffuser 240 can thereby be achieved if the illuminance(e.g., I₁, I₂, I₃) and projected solid angles (e.g. Ω₁, Ω₂, Ω₃) areappropriately matched. L₁, L₂, and L₃ are therefore preferablysubstantially equal.

A plot of the substantially constant luminance at the spatial lightmodulator 236 is shown in FIG. 43. The luminance of the spatial lightmodulator 236 may, for example, be about 10 nits to 150 nits, dependingpossibly on the application and/or system design. These valuescorrespond to the luminance at the eye. Luminance at the LCD arepreferably higher to compensate for losses in the imaging optics. FIG.44 is a histogram of luminance (in nits) that illustrates that theluminous flux per area per steradian values received over the spatiallight modulator 236 are largely similar. The variation in luminance, forexample, may be less than 10% across small regions of the display or 50%between any two points in the display. Different specifications of thevariation may be employed for different applications. For example, insome embodiments, the luminance at the LCD preferably does not vary by afactor greater than about 1.5. The spatial light modulator 236 thereforepreferably appears to have a constant luminance at the differentpositions thereon (assuming the liquid crystal is not modulated toproduce an image or pattern). Absent this combination, the display,projector, or other optical system may appear to the viewer to benon-uniformly lit.

Other configurations for providing non-uniform illumination and uniformluminance may be employed. In FIG. 45, for example, a light pipe 680feeds into a light box 682 optically coupled to a plurality of anglearea converters such as compound parabolic collectors (CPCs) 684disposed across the light box. This light box 682 typically comprises achamber defined, for example, by diffusely reflecting sidewalls ortextured surfaces. Such lightboxes are similar to those used as LCDbacklights for direct view applications. The angle area converters, aredisposed on one of the sidewalls. Twenty exemplary angle area converters684, here compound parabolic collectors 684, are shown. In theembodiment shown, each of these collectors 684 comprises a pair ofparabolic reflectors 686 oppositely situated along an optical axis 688through the respective angle area converters 684. The pair of spacedapart parabolic reflectors 686 define input and output apertures 690,692 and numerical apertures. In the embodiment shown in FIG. 45, theinput apertures 690 and numerical apertures for the plurality of anglearea converters 684 increases with longitudinal position (in the Xdirection) across the light box 682. The numerical aperture alsoincreases although the output aperture is substantially the same for theplurality of angle area converters 684.

FIG. 45 is a cross-sectional view, and thus the sidewalls of the lightbox 682 as well as the angle area converters 686 extend into the Zdirection as well. Accordingly, the angle area converters 684 aresymmetrical about a plane that corresponds to the optical axis 688 shownin the cross-section of FIG. 45. The light box 682 and plurality ofangle area converters 688 are disposed in front of one of the inputfaces of the V prism.

As illustrated by arrows, the light pipe 680 couples light into thelight box 682. This light exits the light box 682 through the pluralityof angle area converters 684. The different numerical apertures anddifferent apertures 690 control the illumination in the lateral (X)direction as well as the projected solid angle into which the light isoutput.

Accordingly, the angle area converters convert increased area at theinput into increased numerical aperture at the output. The increasednumerical aperture at the output is useful for matching to increasingf-number with position across the field. To provide constant luminance,more light is collected with increased input aperture to accommodateincreased numerical aperture at the output.

The compound parabolic collectors work well as angle area converters 684with the light box 682. The luminance into the compound paraboliccollectors equals the luminance out of the compound parabolic reflector.The f-number is controlled by using a different compound parabolicreflector input size. As the input sizes vary across the light box 682,gaps between the CPC prevent light from immediately exiting the lightbox 682, however, this light is reflected back into the light box andrecycled for subsequent egress through the compound paraboliccollectors. Gaps between the output apertures of the CPCs may, however,introduce variation in the “average” spatial luminance across the field.

Accordingly, the plurality of angle area converters 684 can control theillumination that reaches the input face of the V-prism. In certainpreferred embodiments, the illuminance and projected solid angle vary toprovide substantially constant luminance. Although the plurality ofangle area converters 684 may be selected to provide non-uniformilluminance and uniform luminance, other designs are possible whereuniform illuminance and/or non-uniform luminance is provided. Othertypes of configurations may also be employed. Components other thanlight boxes and angle area converters may also be employed in otherembodiments. Other types of angle area converters different fromcompound parabolic collectors may also be employed. A lens arraycomprising a plurality of lenses having increasing numerical aperturemay be employed in certain embodiments.

Implementations for illuminating displays, projectors, and other opticalsystems should not be limited to those embodiments specifically shownherein. For example, the various components specifically described maybe included or excluded and their interrelationship may be altered. Forinstance, configurations for providing non-uniform illumination at thediffuser 240 other than the off-axis scheme depicted in FIG. 38 may beemployed. The diffuser 240 may comprise devices well-known in the artsuch as diffractive optical elements, holographic optical elements,holographic diffusers as well as structures yet to be devised. Also,although embodiments are depicted that include a “V” prism 202 havingtwo ports 208, 210, other beamsplitting elements may be employed and thenumber of input ports need not be limited to two. The system may includeone or more input ports. Other techniques for directing the illuminationonto the spatial light modulator 236 may also be employed as wellalthough polarization beamsplitters 202 such as the “V” prism offer someadvantages. Various configurations and approaches for providingcomposite colored images are possible.

Moreover, controlling the illumination incident on a diffuser 240 havingvariable scattering properties at different locations may be a powerfultool in improving optical properties of displays, projectors, and otheroptical systems. Although described here in connection with providingconstant luminance, the scattering may be adjusted otherwise to providethe desired non-constant luminance profile. Other variations arepossible as well. Accordingly, the illumination and the scattering orlight dispersing features of the diffuser 240 may be different.

An example of a display device 300 such as a helmet mounted display or,more broadly, a head mounted display that includes a polarizationbeamsplitter such as a “V” prism 302 is shown in FIG. 46. The displaycomprises a liquid crystal spatial light modulator 304 proximal the “V”prism 302. An optical path extends from the spatial light modulator 304through the “V” prism 302 and imaging or projection optics 306 andreflects off a combiner 308 to a viewer's eye 310, which includes apupil 312. The combiner 308 folds the image projected by the imagingoptics 306 into the eye 310. The combiner 308 may be at least partiallytransparent such that the viewer can see both the surroundingenvironment 313 as well as the images and patterns created by thespatial light modulator 304. The combiner 308 may comprise, for example,a visor mounted on a helmet. The combiner 308 can be used for headmounted displays that are not transparent such as may be used inimmersive virtual reality. The combiner 308 shown in FIG. 46 issubstantially planar.

A display 300 having a concave combiner 308 is shown in FIG. 47. Thiscombiner 308 has convergent optical power to image the exit pupil of theprojection optics 306 onto the eye pupil 312 of the wearer. Such acombiner 308 may reduce the aperture size and thus the size and weightof the imaging optics 306 as shown. A wide field-of-view may also beprovided with the powered optical combiner 308 as part of an opticalrelay.

A display 300 that projects the image produced by the spatial lightmodulator 304 at (or near) infinity is shown in FIG. 48. An intermediateprojected image 307 is shown located between the projection optics 306and the combiner 308. A virtual image of the projected images 307 isproduced by the combiner 308 at (or near) infinity, e.g., at a largedistance which is comfortable for viewing by the eye 310. Accordingly,the rays (indicated by dashed lines) are depicted as being substantiallycollimated. This combiner 308 may be partially or totally reflective.

A display 300 having a powered on-axis combiner 308 that forms an imageof the exit pupil of the imaging optics 306 at the eye pupil 112 isshown in FIG. 49. A beamsplitter 309 directs the beam from the projectoroptics 306 to the combiner 308. The combiner 308 shown is circularly orrotationally symmetric about the optical axis passing through thecombiner 308. Similarly, a central ray bundle strikes the on-axisoptical combiner 308 at an angle of zero. Another type of on-axiscombiner is flat. The combiners 308 in FIGS. 47 and 48 are off-axiscombiners and are not circularly symmetric about the respective opticalaxes passing therethrough.

On-axis combiners have the advantage of being rotationally symmetricabout the central ray bundle; as a consequence, aberrations introducedby the combiner may be corrected in the projection optics using surfacesthat are also rotationally symmetric about the central ray bundle. Thedrawback of an on-axis combiner is that a beamsplitter is also employed,and thus the configuration is heavier and bulkier.

Off-axis combiners are lightweight; however, because the light reflectsobliquely from a powered reflecting surface, larger amounts ofaberration (chiefly, astigmatism) may be generated in both the image ofthe pupil (see FIG. 47) and in the intended display image (see FIG. 48).To reduce these aberrations, the combiner surface can be made aspheric,for example, as a toroidal surface, anamorphic surface, or other type ofsurface.

Preferably control is provided for both the aberrations of the image aswell as the aberrations of the pupil. If the pupil image issubstantially uncorrected, for example, the caustic (region where therays cross) near the pupil may be large such that large-diameter opticsare preferably used to intercept the rays. In addition, the aberrationsof the pupil are not entirely separable from those of the image. If, forexample, the ray bundles for some of the image field locations havecrossed before reaching the imaging optics, and others have not, thenthe imaging optics are presented with the field positions in a“scrambled” order, and performing image correction may be difficult.

In one preferred embodiment, a combiner having a conic surface and morespecifically an ellipsoid of revolution may be employed. Preferably,this ellipsoid has one of two conic foci located at or near the eye ofthe wearer, and the other conic focus located at or near the pupil ofthe projection optics.

Such a design provides several advantages. Since the conic surface is asurface of revolution, this surface may be fabricated throughsingle-axis diamond turning. If the part is to be made inmass-production using an injection molding, compression molding, orcasting, then the mold inserts may be made by injection molding. Also,if one conic focus is at the eye and the other conic focus is at thepupil of the projection optics, then spherical aberration of the pupilmay be substantially reduced or eliminated. In addition, the centralrays for all the points in the field preferably cross at the center ofthe pupil, and the “scrambling” described above is thereby substantiallyreduced or eliminated. Also astigmatism in the image is reduced, since aconic surface does not introduce astigmatism when one of the foci isplaced at the pupil.

FIG. 50 shows an exemplary display device 400 comprising a spatial lightmodulator 402, a beamsplitter 404 such as a “V” prism for illuminatingthe spatial light modulator, imaging optics 406, and a combiner 408. Thedisplay device 400 may comprise a head-mounted display such as ahelmet-mounted display. Accordingly, the combiner 408 combines imagesformed using the spatial light modulator 402 with the forwardfield-of-view of the wearer's eye. The “V” prism 404 may comprise highindex flint to reduce the size and weight of the system 400. The displaydevice 400 further includes a wedge 409 between the “V” prism 404 andthe spatial light modulator 402 as described above. The wedge 409 maycomprise a high index crown to effectively control the aberrations,while minimizing the size and weight of the system 400. The combiner 408is an “elliptical” combiner conforming to the shape of an ellipsoid(shown in cross-section as an ellipse 414). One of the foci of theellipse is at the stop, which preferably corresponds to the pupil of theeye.

A prescription for one preferred embodiment of the display device 400 ispresented in TABLES I and II wherein the optical parameters for opticalelements A1 to A13 are listed. These optical parameters include radiusof curvature, thickness, material, as well as terms, where appropriate,defining aspheric curvature, tilt, and decenter. The radius ofcurvature, thickness, and decenter data are in millimeters. As is wellknown, aspheric surfaces may be defined by the following expression:Aρ ⁴ +Bρ ⁶ +Cρ ⁸ +Dρ ¹⁰ +Eρ ¹² +Fρ ¹⁴where ρ is the radial dimension. Non-zero values for one or more ofthese constants A, B, C, D, etc. are listed when the surface isaspheric. Additionally, the conic constant, k, may be provided when thesurface is a conic surface. Tilt about the X axis as well as decenter inthe Y and Z directions are also included for some of the surfaces inTABLE II.

The imaging optics 406 comprises ten refractive lenses A2-A11, each ofwhich comprises glass. The imaging optics 406 comprises two groups. Thefirst group comprises the single lens A2. The second group comprises theremaining lenses, A3-A11. The field aberrations from the ellipticalcombiner A1 are partially cancelled by the lenses A2 in the first group,which is a low index meniscus lens and which does not share the axis ofthe group of lenses A3-A10 in the second group or of the combiner. Inparticular, the meniscus lens A2 is tilted and/or decentered withrespect to the remainder of the lenses A3-A11 in the optical system andthe V-prism A12. Accordingly, this tilted lens A2 has a first opticalaxis about which the lens is circularly symmetric. Similarly, theplurality of lenses A3-A11 in the second group has a correspondingsecond optical axis about which the lenses are circularly symmetric. Thetwo optical axes, however, are different and non-parallel. Preferably,only one lens (in the first group) is tilted with respect to the otherlenses (in the second group) although in other embodiments the firstgroup comprises more than one lens aligned along the first optical axis.

One of these lenses A4 comprising the imaging optics 406 has an asphericshaped surface. This aspheric surface is near an intermediate pupil toprovide for spherical aberration correction. Color correction isprovided by the cemented doublets A5/A6, A8/A9, and A10/A11.

The entrance pupil diameter for this system is 15.0 millimeters. Thefield-of-view is evaluated between 50 to −15 degrees along thehorizontal axis and 25 to −25 degrees along the vertical axis. Theimaging optics 406 has an exit pupil that is imaged by the combiner 408to form a conjugate pupil 412 where the eye pupil (not shown) may beplaced.

FIG. 51 shows another embodiment of the display device 400. Aprescription for one preferred embodiment of this display device 400 ispresented in TABLES III and IV. The optical parameters for nine opticalelements B1 to B9 are listed. One of the optical elements B1 correspondsto the reflective combiner 408. One of the optical elements B8corresponds to the V-prism 408, and one of the optical elements B9corresponds to the wedge 410. The imaging optics 406 comprises theremaining six optical elements B2-B7, each a refractive lens. Theimaging optics 406 is split into a first group comprising the first lensB2 and a second group comprising the remaining five lenses B3-B7.

Like the system 400 in FIG. 50, the combiner 408 is an “elliptical”combiner conforming to the shape of an ellipsoid (shown in cross-sectionas an ellipse 414). In this embodiment, however, two of the lenses B3and B6 are plastic. These elements comprise Zeonex 1600R (Z-1600R)available from Zeon Chemicals L.P., Louisville, Ky. Plastic lenses canbe fabricated in high volumes at lower cost than glass lenses. Plasticlenses are also lighter. The remaining refractive optical components B2,B4, B5, B7, B8, B9, comprise optical glass. The “V” prism 404 (B8)comprises high index flint to reduce the size and weight of the system400. The wedge 409 between the “V” prism 404 and the spatial lightmodulator 402 comprises high index crown to effectively control theaberrations, while minimizing the size and weight of the system 400.Both of the plastic lenses B3, B6 have aspheric surfaces. One of thelenses B2 is also tilted and decentered with respect to the other lensesB3-B9. Like the system 400 in FIG. 51, the lens in the first group B2, ameniscus lens, is symmetrical about a first optical axis. The remaininglenses B3-B9, which are in the second group, are symmetrical about asecond optical axis. These two optical axes, however, are different.Advantageously, this optical system also has only nine optical elementsB1-B9, six of which are lenses. The imaging system 406 comprises acemented doublet B4/B5 for color correction. The aspheric surface on B6is near the “V” prism to correct for astigmatism and coma. The asphericsurface on B3 is near an intermediate pupil to provide for sphericalaberration correction. The field aberrations from the ellipticalcombiner B1 are partially cancelled by the low index meniscus lens B2which, as discussed above, does not share the axis of the first group oflenses B3-B7 nor that of the combiner. Some of the edges of a number ofthe lenses B3, B4, B6, B7, are cut off to reduce the weight of thesystem 400. The entrance pupil diameter for this system is 15.0millimeters. The field-of-view is evaluated between 50 to −15 degreesalong the horizontal axis and 25 to −25 degrees along the vertical axis.

FIG. 52 shows another embodiment of the display device 400. Aprescription for one preferred embodiment of this display device 400 ispresented in TABLES V and VI. This optical system has a reduced numberof optical elements. The optical parameters for nine optical elements B1to B9 are listed. One of the optical elements B1 corresponds to thereflective combiner 408. One of the optical elements C6 corresponds tothe V-prism 408, and one of the optical elements C7 corresponds to thewedge 410. The imaging optics 406 comprises the remaining four opticalelements C2-C5, each a refractive lens. This decreased number of lensC2-C5 advantageously reduces the weight and cost of the optical system400. The lenses C2-C5 are grouped into a first group and a second group.The first group comprises the first lens C2 and the second groupcomprises the three remaining lenses C3-C5. In other embodiments, thefirst group may comprise more than one lens, although a single lenselement is preferred.

Like the systems 400 in FIGS. 50 and 51, the combiner 408 is an“elliptical” combiner conforming to the shape of an ellipsoid (shown incross-section as an ellipse 414). In this embodiment, however, each ofthe four powered elements C2-C5 is plastic. These elements C2-C5comprise acrylic (PMMAO), Zeonex 480R (Z-480R), and Zeonex 1600R(Z-1600R). Z-480R and Z-1600R are available from Zeon Chemicals L.P.,Louisville, Ky. Other plastic and non-plastic materials may be used aswell. Plastic lenses, however, can advantageously be fabricated in highvolumes at lower cost than glass lenses. Plastic lenses are alsolighter. The “V” prism comprises a high index flint to reduce the sizeand weight of the system. The wedge between the “V” prism 404 and thespatial light modulator 402 comprises a high index crown to effectivelycontrol the aberrations, while minimizing the size and weight of thesystem.

Each of the lenses C2-C5 in the imaging system is aspheric to correctfor monochromatic aberrations. One of the lenses C2 is also tilted anddecentered with respect to the other three lenses C3-C5. Like the system400 in FIGS. 50 and 51, the lens C2 in the first group, a meniscus lens,is symmetrical about a first optical axis. The remaining lenses C3-C5,which are in the second group, are also symmetrical about a secondoptical axis. The first and second optical axes are orienteddifferently. The optical elements C3-C5 in the second group eachcomprises a plastic flint. One lens C4 in the second group comprises adiffractive element for color correction. This diffractive element, ahologram, is characterized by the following expression:φ=c ₁ρ² +c ₂ρ⁴where φ is the phase shift imparted on the wavefront passing through thediffractive features on this optical element C4, ρ is the radialdimension, and c₁ and c₂ are constants. The values of c₁ and c₂ are−7.285×10⁻⁴ and −1.677×10⁻⁷, respectively. The diffractive opticalelement is designed to use the first order (m=⁺1) at a wavelength ofabout 515 nanometers. The field aberrations from the elliptical combinerare partially cancelled by the low index lens in the first group, whichdoes not share the same optical axis as either of the second group oflenses in the imaging optics 406 or of the combiner 408. The entrancepupil diameter for this system is 15.0 millimeters. The field-of-view isevaluated between 50 to −15 degrees along the horizontal axis and 25 to−25 degrees along the vertical axis.

Other designs may be used as well. For example, variations in thenumber, shape, thickness, material, position, and orientation, arepossible. Holographic or diffractive optical elements, refractive and/orreflective optical elements can be employed in a variety ofarrangements. Many other variations are possible and the particulardesign should not be limited to the exact prescriptions included herein.

Various preferred embodiments, however, employ combiners having a shapein the form of a conic surface. Conic surfaces are formed by generatinga conic section, a particular type of curve, and rotating the curveabout an axis to sweep out a three-dimensional surface. The shape of aconic surface is determined by its conic constant, k. The conicconstant, k, is equal to the negative of the square of the eccentricity,e, of the conic curve in two dimensions that is rotated to form thethree-dimensional surface. Conic surfaces are well know and aredescribed, for example, in “Aspheric Surfaces”, Chapter 3 of AppliedOptics and Optical Engineering, Vol. VIII, R. Shannon and J. Wyant, ed.,Academic Press, New York N.Y. 1980.

An ellipsoid (also known as a prolate spheroid) is formed by rotating anellipse about an axis, referred to as a major axis, which joins twoconic foci. The conic constant for an ellipsoid has a value between zeroand −1. A sphere is a special case of an ellipsoid, with a conicconstant of zero. A hyperboloid is formed in a similar manner, however,the value of the conic constant is more negative than −1. A paraboloidhas a conic constant of exactly −1, and is formed by rotating a parabolaabout an axis that is perpendicular to a line referred to as a directrixof the parabola and a point on the axis, the focus of the parabola. Anoblate spheroid has a positive conic constant. In various preferredembodiments, the conic constant is between about −0.25 and 0, or 0 and−0.60, and may be between about −0.36 and 0, or 0 and −0.44.

In various preferred embodiments for eliminating spherical aberration ofthe pupil, one conic focus 418 is located exactly at the eye 412 and theother conic focus 420 is located exactly at the pupil 416 of theprojection optics 406. The conic constant for this combiner 408 has aconic constant between 0 and −1 and the surface is thereforeellipsoidal. (Since the eye pupil and the projection optics pupil arephysically separated, the surface is not spherical.)

FIG. 53 is a schematic cross-sectional representation of the ellipsoid(shown as an ellipse 414) and the combiner 408 substantially conformingto the shape of the ellipsoid. The ellipsoid includes two foci 418, 420and a major axis 422 through the two foci. A pupil 412 in the viewer'seye and an exit pupil 416 for the imaging optics 406 are depicted at thetwo foci 418, 420 of the ellipsoid. In various embodiments, the shape ofthe combiner 408 substantially conforms to a portion of the ellipsoid414. In addition, the ellipsoid 414 is positioned with respect to thepupil 412 of the eye and the exit pupil 416 of the imaging optics 406such that the pupils 412, 416 substantially coincide with the locationsof the foci 418, 420 of the corresponding ellipsoid defining the shapeof the combiner 408. In such a configuration, the ellipsoidal combiner408 preferably images the projector pupil 416 generally onto the eyepupil 412.

FIG. 54 illustrates another example wherein the combiner 408 conforms tothe shape of an ellipsoid and the pupil 412 of the viewer's eye and theexit pupil 416 of the imaging optics 406 substantially correspond to thelocations of the foci 418, 420 of the ellipsoid. FIG. 54 also depicts aplurality of lenses comprising the imaging or projection optics 406. Theshape of the combiner 408 may deviate from conforming to a portion of anellipse 414 and the pupils 412, 416 may be shifted with respect to thefoci 418, 420. The major axis 422 of the ellipsoid 414 intersects thetwo foci 418, 420. As shown by the location of beam path reflected fromthe combiner 408 with respect to the major axis 422 through theellipsoid, the combiner is an off-axis combiner.

In one preferred embodiment, to eliminate spherical aberration at thecenter of the field-of-view, a reflective surface having a shape of aparaboloid (formed by rotating a parabola about its axis of symmetry)may be used. Preferably, this rotation axis of the paraboloid definingthe reflective surface is substantially parallel to the line-of-sight ofthe eye at the center of the field. Moreover, the conic focus to theparaboloid is preferably disposed at the image point for that field.

FIG. 55, for example, illustrates another display system 450 comprisingan object plane 454, imaging optics 456, and a combiner 458. An opticalpath extends from the object plane 454, through the imaging optics 456,off the combiner 458 and into an eye 460 with a pupil 462. FIG. 55depicts a schematic cross-sectional representation of a paraboloid(shown as a parabola 464) and the combiner 458 substantially conformingto the shape of the paraboloid. The paraboloid 464 is defined by a focus466 and a directrix 468. An intermediate image 467 is at the focus 466of the parabola 464. In various embodiments, the shape of the combiner458 substantially conforms to a portion of a paraboloid 464.Additionally, the parabola 464 is positioned such that the focus 466 ofthe paraboloid 464 defining the shape of the combiner 458 substantiallyoverlaps the intermediate image 467. With such a configuration, theintermediate image 467 is reproduced at or near infinity, e.g., adistance sufficiently far for comfortable viewing of the viewer, asclose as several meters to several kilometers as well as outside thisrange. As discussed above, spherical aberration at the pupil 462 may bereduced with this configuration.

In some embodiments, the goals of simultaneously reducing theaberrations at the pupil and the aberration at the image lead to a conicconstant between 0 and −1, which yields an ellipsoid. The conic foci ofthis ellipsoid are preferably located near, although not coincidentwith, the eye and the projection optics pupil, respectively. Theproximity in relationship with the foci may be selected so as to reducepupil and image aberration, e.g., as reflected in a merit function usedto evaluate different designs. In various preferred embodiments, theexit pupil is at a distance from the one of the foci that is less thanabout ¼ the distance along the major axis of the ellipsoid thatseparates the foci.

FIG. 56, for example, shows an embodiment wherein the combiner 408comprises an ellipsoidal surface 414 and the viewer's eye and the exitpupil 416 of the imaging optics 406 are shifted away from the foci 418,420 of the ellipse defining the shape of the combiner. Morespecifically, one of the foci 420 is between the exit pupil 416 of theimaging optics 406 and an intermediate image 407 formed by the imagingoptics. The combiner 408 is positioned with respect to the imagingoptics 406 and the object 404 as well as the resultant intermediateimage 407 to project the intermediate image to or near infinity (e.g., adistance sufficiently far for comfortable viewing of the viewer, asclose as several meters to kilometers). Accordingly, the rays (indicatedby dashed lines) are depicted as being substantially collimated Inaddition, both the aberration at the pupil and the aberration at theimage are reduced. The distance of the eye and pupil of the projectionoptics is preferably such that reduced value of the image and pupilaberrations is obtained.

Although various structures and methods for illumination and imaging aredepicted in connection with displays such as head mounted displays andhelmet mounted displays, other displays such as heads-up displays aswell as non-display applications can benefit from the use of suchtechnology. Examples of devices that may incorporate this technologyinclude projectors, flat-panel displays, back-projection TV's, computerscreens, cell phones, GPS systems, electronic games, palm tops, personalassistants and more. This technology may be particularly useful foraerospace, automotive, and nautical instruments and components,scientific apparatus and equipment, and military and manufacturingequipment and machinery. The potential applications range from homeelectronics and appliances to interfaces for business and industrialtools, medical devices and instruments, as well as other electronic andoptical displays and systems both well known as well as those yet to bedevised. Other applications, for example, in industry, such as formanufacturing, e.g., parts inspection and quality control, are possible.The applications should not be limited to those recited herein. Otheruses are possible.

Similarly, configurations other than those described herein arepossible. The structures, devices, systems, and methods may includeadditional components, features, and steps and any of these components,features, and steps may be excluded and may or may not be replaced withothers. The arrangements may be different.

Moreover, various embodiments of the invention have been describedabove. Although this invention has been described with reference tothese specific embodiments, the descriptions are intended to beillustrative of the invention and are not intended to be limiting.Various modifications and applications may occur to those skilled in theart without departing from the true spirit and scope of the invention asdefined in the appended claims.

TABLE I Elements Surface Radius Thickness Glass Image  0 InfinityInfinity Stop  1 Infinity 0.000 A1  2 (aspheric) −91.077 0.000Reflective  3 (tilt/decenter) Infinity −10.295 A2  4 29.791 −3.246 NBK10 5 31.398 0.000  6 (tilt/decenter) Infinity −1.076 A3  7 −51.916 −7.348SFL57  8 −84.361 −3.630 A4  9 (aspheric) −80.585 −10.299 NBK10 10136.780 −0.100 A5 11 −63.316 −1.200 SFL57 A6 12 −33.076 −17.828 NBK7 1361.314 −0.100 A7 14 72.798 −1.360 SFL57 15 −3385.379 −0.100 A8 16−73.456 −12.863 NLAK33 A9 17 58.037 −2.475 NBK10 18 −111.010 −0.998 A1019 −89.176 −1.205 NSF5 A11 20 −32.741 −12.929 NLAK33 21 −159.940 −2.190A12 22 Infinity −11.000 SPF57 A13 23 Infinity −5.000 NLAK33 24 (tilt)Infinity 0.000 25 (tilt) Infinity −1.023 Object 26 Infinity 0.000

TABLE II Element Surface Aspheric Coefficients Tilt & Decenter A1 2Conic Const. −0.363 Tilt X −86.59° A2 3 Tilt X −69.56° Decenter Y155.558 Decenter Z −5.097 A3 6 Tilt X −12.65° Decenter Y −4.504 A4 9 A0.432 × 10⁻⁵ B 0.700 × 10⁻⁰⁹ A13 24 Tilt X 8.66° 25 Tilt X −3.84°Decenter Y −16.260

TABLE III Elements Surface Radius Thickness Glass Image  0 InfinityInfinity Stop  1 Infinity 0.000 B1  2 (aspheric) −89.775 0.000Reflective  3 (tilt/decenter) Infinity 0.000 B2  4 60.718 −3.000 NBK10 5 134.450 0.000  6 (tilt/decenter) Infinity −15.482 B3  7 (aspheric)−42.156 −18.000 Z-1600R  8 (aspheric) 112.611 −11.423 B4  9 −38.117−13.000 SK51 B5 10 77.117 −3.200 SFL57 11 −57.234 −0.271 B6 12 −39.687−10.000 Z-1600R 13 (aspheric) 184.200 −1.110 B7 14 −33.701 −12.291 NSK515 −169.515 −1.808 B8 16 Infinity −11.000 SKL57 B9 17 Infinity −4.500NLAK33 18 (tilt) Infinity 0.000 19 (tilt/decenter) Infinity −1.138Object 20 Infinity 0.000

TABLE IV Element Surface Aspheric Coefficients Tilt & Decenter B1 2Conic Const. −0.354 Tilt X −74.86° B2 3 Tilt X −56.49° Decenter Y142.230 Decenter Z −33.024 B3 6 Tilt X −4.58° Decenter Y −4.261 B3 7 A0.104 × 10⁻⁵ B −0.323 × 10⁻⁰⁹ B3 8 A −0.243 × 10⁻⁵ B −0.186 × 10⁻⁰⁹ B713 A −0.426 × 10⁻⁵ B −0.358 × 10⁻⁰⁸ C 0.313 × 10⁻¹¹ D 0.820 × 10⁻¹⁵ B918 Tilt X 7.34° 19 Tilt X −7.67° Decenter Y −11.883

TABLE V Elements Surface Radius Thickness Glass Image  0 InfinityInfinity Stop  1 Infinity 0.000 C1  2 (aspheric) −92.177 0.000Reflective  3 (tilt/decenter) Infinity 0.000 C2  4 (aspheric) 234.958−5.000 PMMAO  5 −207.944 0.000  6 (tilt/decenter) Infinity −11.044 C3  7(aspheric) −40.907 −22.000 Z-480R  8 126.927 −12.370  9 77.117 −3.200 C410 −39.049 −9.000 Z-480R 11 (aspheric) −846.922 −12.954 (holographic) C512 (aspheric) −36.352 −17.180 Z-480R 13 −467.961 −0.100 C6 14 Infinity−11.000 SKL57 C7 15 Infinity −4.500 NLAK33 16 (tilt) Infinity 0.000 17(tilt/decenter) Infinity −1.011 Object 18 Infinity 0.000

TABLE VI Element Surface Aspheric Coefficients Tilt & Decenter C1 2Conic Const. −0.325 Tilt X −61.36° C2 3 Tilt X −44.95° Decenter Y118.396 Decenter Z −63.306 C2 4 A 0.535 × 10⁻⁶ B 0.216 × 10⁻⁸ C −0.133 ×10⁻¹¹ D 0.723 × 10⁻¹⁵ C3 6 Tilt X −4.23° Decenter Y −1.040 C3 7 A 0.198× 10⁻⁵ B −0.397 × 10⁻⁰⁹ C 0.451 × 10⁻¹² D 0.272 × 10⁻¹⁵ C4 11 A −0.521 ×10⁻⁵ B −0.739 × 10⁻⁰⁹ C 0.256 × 10⁻¹¹ D −0.920 × 10⁻¹⁴ c1 −7.285 × 10⁻⁴c2 −1.677 × 10⁻⁷ C5 12 A −0.934 × 10⁻⁶ B −0.944 × 10⁻⁰⁹ C 0.697 × 10⁻¹²D −0.170 × 10⁻¹⁴ C7 16 Tilt X 7.92° 17 Tilt X −8.24° Decenter Y −10.884

1. A head mounted display for displaying images comprising: a spatiallight modulator comprising a plurality of pixels selectively adjustablefor producing spatial patterns; a light source for illuminating saidspatial light modulator; imaging optics disposed with respect to thespatial light modulator to receive light from said spatial lightmodulator for forming an image thereof; an aspheric reflective combinersubstantially conforming to the shape of a portion of a surface thatcomprises an ellipsoid defined by first and second foci, said asphericcombiner reflecting light from said imaging optics such that said imagemay be displayed; and head gear for supporting said spatial lightmodulator, imaging optics, and combiner; wherein said imaging optics hasan exit pupil substantially coincident with said first focus of saidellipsoid.
 2. The head mounted display of claim 1, wherein said headgear comprises a helmet.
 3. The head mounted display of claim 1, whereinsaid imaging optics comprises a plurality of lens elements.
 4. A headmounted display for displaying images comprising: a spatial lightmodulator comprising a plurality of pixels selectively adjustable forproducing spatial patterns; a light source for illuminating said spatiallight modulator; imaging optics disposed with respect to the spatiallight modulator to receive light from said spatial light modulator forforming an image thereof; an aspheric reflective combiner substantiallyconforming to the shape of a portion of a surface that comprises anellipsoid defined by first and second foci, said aspheric combinerreflecting light from said imaging optics such that said image may bedisplayed; and head gear for supporting said spatial light modulator,imaging optics, and combiner; wherein said imaging optics has an exitpupil and is disposed with respect to said aspheric combiner to form anintermediate image between said exit pupil and said aspheric combiner.5. The head mounted display of claim 4, wherein said imaging opticscomprises a plurality of lens elements.
 6. The head mounted display ofclaim 4, wherein said head gear comprises a helmet.
 7. A head mounteddisplay for displaying images comprising: a spatial light modulator; alight source for illuminating said spatial light modulator; first opticsdisposed with respect to the spatial light modulator to receive lightfrom said spatial light modulator; second optics comprising anellipsoidal combiner having a reflective surface comprising anellipsoidal surface curved in two dimensions in accordance with thecurvature of an ellipsoid, said ellipsoidal combiner reflecting lightreceived from said first optics; and head gear for supporting saidspatial light modulator and first and second optics; wherein saidellipsoidal reflective surface comprises an off-axis portion of saidellipsoid spaced from and not intersecting major and minor axes of saidellipsoid.
 8. A head mounted display for displaying images comprising: aspatial light modulator; a light source for illuminating said spatiallight modulator; first optics disposed with respect to the spatial lightmodulator to receive light from said spatial light modulator; secondoptics comprising an ellipsoidal combiner having a reflective surfacecomprising an ellipsoidal surface curved in two dimensions in accordancewith the curvature of an ellipsoid, said ellipsoidal combiner reflectinglight received from said first optics; and head gear for supporting saidspatial light modulator and first and second optics; wherein saidellipsoid has first and second foci and said first optics has an exitpupil substantially located at said first focus.
 9. A head mounteddisplay for displaying images at a wearer's eye comprising: a spatiallight modulator; a light source for illuminating said spatial lightmodulator; first optics disposed with respect to the spatial lightmodulator to receive light from said spatial light modulator; secondoptics comprising an ellipsoidal combiner having a reflective surfacecomprising an ellipsoidal surface curved in two dimensions in accordancewith the curvature of an ellipsoid, said ellipsoidal combiner reflectinglight received from said first optics; and head gear for supporting saidspatial light modulator and first and second optics; wherein saidellipsoid has first and second foci and said first optics has an exitpupil located in sufficient proximity to said first focus to reduceaberration at said exit pupil but is non-coincident with and separatedfrom said first focus to reduce aberration in said images at saidwearer's eye.
 10. A head mounted display for displaying imagescomprising: a spatial light modulator; a light source for illuminatingsaid spatial light modulator; first optics disposed with respect to thespatial light modulator to receive light from said spatial lightmodulator; second optics comprising an ellipsoidal combiner having areflective surface comprising an ellipsoidal surface curved in twodimensions in accordance with the curvature of an ellipsoid, saidellipsoidal combiner reflecting light received from said first optics;and head gear for supporting said spatial light modulator and first andsecond optics; wherein said ellipsoid has first and second foci and saidfirst optics has an exit pupil and is disposed with respect to saidellipsoidal combiner to form an intermediate image between said exitpupil and said ellipsoidal combiner.
 11. A head mounted display fordisplaying images comprising: a spatial light modulator; a light sourcefor illuminating said spatial light modulator; first optics disposedwith respect to the spatial light modulator to receive light from saidspatial light modulator; second optics comprising an ellipsoidalcombiner having a reflective surface comprising an ellipsoidal surfacecurved in two dimensions in accordance with the curvature of anellipsoid, said ellipsoidal combiner reflecting light received from saidfirst optics; and head gear for supporting said spatial light modulatorand first and second optics; wherein said ellipsoid has first and secondfoci and said head gear is configured to position a wearer's eye inproximity to said second focus to reduce aberration at said pupil butnon-coincident with the second focus to reduce aberration at the eye.12. A head mounted display for displaying images comprising: a spatiallight modulator; a light source for illuminating said spatial lightmodulator; first optics disposed with respect to the spatial lightmodulator to receive light from said spatial light modulator; secondoptics comprising an ellipsoidal combiner having a reflective surfacecomprising an ellipsoidal surface curved in two dimensions in accordancewith the curvature of an ellipsoid, said ellipsoidal combiner reflectinglight received from said first optics; and head gear for supporting saidspatial light modulator and first and second optics; wherein saidellipsoid has first and second foci and said head gear is configured toposition a wearer's eye between said second focus and said ellipsoid.13. The head mounted display of claim 12, wherein said ellipsoid has amajor axis between about 100 and 500 millimeters in length, and a minoraxis between about 100 and 500 millimeters in length.
 14. A head mounteddisplay for displaying images comprising: a spatial light modulator; alight source for illuminating said spatial light modulator; first opticsdisposed with respect to the spatial light modulator to receive lightfrom said spatial light modulator; second optics comprising anellipsoidal combiner having a reflective surface comprising anellipsoidal surface curved in two dimensions in accordance with thecurvature of an ellipsoid, said ellipsoidal combiner reflecting lightreceived from said first optics; and head gear for supporting saidspatial light modulator and first and second optics; wherein saidellipsoid has a major axis between 100 and 500 millimeters in length,and a minor axis between about 100 and 500 millimeters in length. 15.The head mounted display of claim 14, wherein said major axis is betweenabout 300 and 400 millimeters in length.
 16. The head mounted display ofclaim 14, wherein said minor axis is between about 200 and 300millimeters in length.
 17. A method of forming an image in the eye of awearer of a head mounted display, said method comprising: illuminating aspatial light modulator in said head mounted display; collecting lightfrom said spatial light modulator with projection optics having an exitpupil; and reflecting said collected light from an ellipsoidal reflectorinto the eye of the wearer to form an image at the eye, said ellipsoidalreflector having a shape substantially conforming to a portion of anellipsoid; wherein said ellipsoidal reflector has first and second fociand said exit pupil of said projection optics is disposed in closeproximity to but non-coincident with said first focus thereby reducingaberration at said exit pupil.
 18. The method of claim 17, wherein saidfirst focus and said second focus are separated by a distance and saidexit pupil of said projection optics is disposed from said first focusby less than about ¼ the distance separating said first focus and saidsecond focus.
 19. The method of claim 17, wherein said exit pupil ofsaid projection optics is disposed within less than about 70 millimetersfrom said first focus thereby reducing aberration at said exit pupil.20. A method of forming an image in the eye of a wearer of a headmounted display, said method comprising: illuminating a spatial lightmodulator in said head mounted display; collecting light from saidspatial light modulator with projection optics having an exit pupil;reflecting said collected light from an ellipsoidal reflector into theeye of the wearer to form an image at the eye, said ellipsoidalreflector having a shape substantially conforming to a portion of anellipsoid; and forming an intermediate image with said projection opticsbetween said exit pupil of said projection optics and said ellipsoidalreflector.
 21. The method of claim 20, wherein a focus of said ellipsoidis between said exit pupil of said projection optics and saidintermediate image thereby reducing aberration at said exit pupil andthereby reducing aberration of said image in said eye.